RO/DI system? Why????

Why all the need for RO/DI systems?



In the course of keeping our home aquariums we often find we are told to make sure that we have a good Reverse Osmosis/Deionization system in place and to use the water generated for our aquariums rather than tap water.  When we begin the hobby we often ask ourselves why do we need o spend the extra money for a system that just cleans the water we use up a bit.  Well there are many good reasons for getting a system for use with our aquariums as well as for our own personal use.


First of all let’s talk about a couple things that are very harmful for fish and can cause irritation to some people, Chlorine and Chloramines. 


Now most people know what chlorine is, especially if they have been swimming at a public or private pool in the majority of the US or elsewhere.  Chlorine is used as a disinfectant so basically it kills harmful bacteria or viruses that can cause problems in the water.  It also can cause a few problems when you go swimming, some people’s hair will turn a greenish tint and if the levels are not correct it can cause your eyes to sting and burn.  In aquatic life chlorine has a much more detrimental effect, death.  You see humans can ingest chlorine with no ill effect because it never really interacts with our bloodstream.  You can cook with it, wash with it, bathe with it even with a cut or scrape and have no problems at the levels that are mandated safe fro our drinking water supplies.  But fish and other marine life are a bit different, they use the water to take in oxygen and their gills act almost like our lungs and have a very thin wall that separates the water and their blood so chlorine is able to pass directly into their bodies.  The same thing can actually happen to people who are using kidney dialysis, as their kidney’s can not filter their blood a membrane is put in place that allows their blood to flow in very close contact with water which allows toxin’s to exchange from the blood to the water.  If the water used is not free of chemicals and metals the exchange can also work in reverse and ultimately cause death.


The other major disinfectant used in the US is chloramines now it may sound very close to the same as chlorine and in fact does have chlorine as a part of the chemical makeup but in some ways is much more dangerous.  Chlorine will dissipate from water if it is left in direct sunlight for about four hours as the sunlight and heat will help to break it down.  You can also boil chlorinated water and the chlorine will break up and become basically harmless.  To do the same things with chloramines you would have to wait well over a week for it to break down in sunlight and have to boil the water for much longer to break it down that way.  So what is this tough stuff?  Well like I said it is chlorine but also has a very small amount of ammonia added to it.  Now if you do the same thing at home by say adding bleach and ammonia together to clean a toilet you will come across some very major problems as it will create a poisonous gas that will quickly make you sick and with enough exposure can kill you.  But in a water treatment plant this reaction is much smaller as it is added into a very large amount of water and protection is used by the technician who adds the chemicals to the water to prevent harm.


So how do we remove these and other nasties from the water?


Well the removal in large scale operations will generally include huge swimming pool sized vats and the addition of chemicals that break up the chloramines bonds and then exposure to sunlight and air to allow the ammonia to offgas (dissipate into the air) and then sunlight to break up the chlorine.  In our homes we generally do not have the space to do things this way so we use activated carbon and it does an excellent job of removing chlorine but chloramines are a bit tougher and the cartridges used in many systems will not last as long unless they are designed for chloramines use.  But I will chat a bit more about that later.


The next question is why chlorine or chloramines is added to the water to begin with and why use chloramines rather than chlorine.


This is a two part question because of a couple things.  First of all the US government has determined that our water system has to stay disinfected to the point of use.  What that means is that the water has to have a disinfectant at a level that will kill any contaminants has to be in place when any person using that water system turns on their faucet.  Some countries just require the water to stay disinfected at the treatment plant and don’t worry so much about the water lines or point of use.  So the best way to keep water disinfected in the US is to have a chemical that will travel with the water to the point is used and the most commonly used solutions are chlorine and chloramines. 


The reason why chloramines are used rather than chlorine is really a pretty simple one though, Cost.  It costs quite a bit of money to disinfect the water systems and the same thing that makes chlorine a better solution for people who have aquariums, the fact that it is easier to remove and will break down faster, makes it more costly for the system operators as it takes higher levels to make it stay in the water system at a high enough level to meet government guidelines.  Chloramines last longer in the system and require less of a total amount to keep them in the water system and a proper level therefore keeping the cost of the system operator down.


Another question to ask is what does my local water system use to disinfect the water?  Well this one is hit or miss.  But generally the larger the overall water system the more likely they are to use chloramines.  The best way to find out is to call your water supplier and ask if they use chlorine or chloramines to disinfect the water supply.  If the person you talk to does not know ask to speak with someone who does.



Now on to the big question, why a RO/DI system?


Whether or not you are using well water or municipal water it is still a good idea to use an RO system.  Both systems can have contaminates that you may not want to drink and both can have chemicals that can wreak havoc with your aquarium.  Also buying filtered water can become costly especially if you have a large system.  Lets say that you are going to a local store and picking up water for between .37 and .50 a gallon maybe you have been really lucky to find refill water at .25 per gallon.  The water that you get from your local supplier will be much cheaper, like Oklahoma City where I live will be about 2.35 per 1000 gallons or .00235 per gallon or .02 per 10 gallons.  So anyway the cost is much higher for buying or even refilling water jugs. Well lets do a little comparison shall we?


I will figure the RO system at 25% efficiency which means we will use 4 gallons of tap water to make 1 gallon of pure water and since the cost of the membranes and cartridges will vary as will times to be replaced I will make them a nice round .02 per gallon of pure water plus the cost of the 4 gallons of water.

Tap per 1000 Tap water RO/DI Bottled water 1 Bottled water 2 Bottled water refill1 Bottled water refill 2
2.35 $0.0024 $0.03 $0.60 $0.50 $0.37 $0.25
100 $0.24 $2.94 $60.00 $50.00 $37.00 $25.00
200 $0.47 $5.88 $120.00 $100.00 $74.00 $50.00
300 $0.71 $8.82 $180.00 $150.00 $111.00 $75.00
400 $0.94 $11.76 $240.00 $200.00 $148.00 $100.00
500 $1.18 $14.70 $300.00 $250.00 $185.00 $125.00
600 $1.41 $17.64 $360.00 $300.00 $222.00 $150.00
700 $1.65 $20.58 $420.00 $350.00 $259.00 $175.00
800 $1.88 $23.52 $480.00 $400.00 $296.00 $200.00
900 $2.12 $26.46 $540.00 $450.00 $333.00 $225.00
1000 $2.35 $29.40 $600.00 $500.00 $370.00 $250.00
1100 $2.59 $32.34 $660.00 $550.00 $407.00 $275.00
1200 $2.82 $35.28 $720.00 $600.00 $444.00 $300.00
1300 $3.06 $38.22 $780.00 $650.00 $481.00 $325.00
1400 $3.29 $41.16 $840.00 $700.00 $518.00 $350.00
1500 $3.53 $44.10 $900.00 $750.00 $555.00 $375.00
1600 $3.76 $47.04 $960.00 $800.00 $592.00 $400.00
1700 $4.00 $49.98 $1,020.00 $850.00 $629.00 $425.00
1800 $4.23 $52.92 $1,080.00 $900.00 $666.00 $450.00
1900 $4.47 $55.86 $1,140.00 $950.00 $703.00 $475.00
2000 $4.70 $58.80 $1,200.00 $1,000.00 $740.00 $500.00
2100 $4.94 $61.74 $1,260.00 $1,050.00 $777.00 $525.00
2200 $5.17 $64.68 $1,320.00 $1,100.00 $814.00 $550.00
2300 $5.41 $67.62 $1,380.00 $1,150.00 $851.00 $575.00
2400 $5.64 $70.56 $1,440.00 $1,200.00 $888.00 $600.00
2500 $5.88 $73.50 $1,500.00 $1,250.00 $925.00 $625.00



So as you can see it can get quite costly to buy the water even if you can refill for .25 per gallon which I have only seen very rarely anymore.  And as an added advantage you can use the RO water as your drinking and icemaker water as well.  So I probably use right around 200 gallons per month of RO water for drinking and tank water for my small system.  And even if you did use just tap water and used a chemical chlorine/chloramine remover you would likely spend at least as much per gallon as the RO/DI system though likely more and would still have the problem with other contaminates in the water.  So the payoff really is there and does exist.  Plus you do not know the quality of the water you will actually receive from these sources if they had a filter malfunction during the production run or if the refill equipment has been properly maintained.


Now lets talk about some of what an RO system removes before the DI resin.


  1. Arsenic
  2. Chloramines/Chlorine
  3. Fluoride
  4. Heavy metals – iron, lead, copper, etc.
  5. Nitrates
  6. Phosphates
  7. Sediment


Many more things are actually removed but this list is some of the more unsavory things that can be in the water.  And what little the RO membrane does not remove the DI resin will likely pick up.  My tap water runs with a Total Dissolved Solids or TDS of about 350 to 450 and just the RO alone will make the TDS around 20 for my system after running for just a couple minutes.  And the average tap water in the use ranges from about 170 to 415 TDS with 500 being the maximum contamination level allowed in the US by the EPA and 1000 being the highest the world health organization deems safe for human consumption.  Even just carbon filtration can help to bring TDS down but to get the water to as pure a level as possible you really want to use an RO/DI system so that you know that the water is as pure as possible before adding it to your system.  The more TDS that is in the starting water the more chemical contaminants that can be found and the harder it will be to keep your tank stable.


But I have well water so no Chlorine or Chloramines.


While it is great to not have to worry about the disinfectants that are used in municipal water systems you may have other things to worry about that is not as common in the cities and rural water districts.  One major thing is often times well water is very high in iron and other metals which are still toxic to marine life and you will have greater chances of having high phosphates and nitrates especially if you are near any farming or ranching activates.  So you still have a need to remove those contaminants from your water and you may even have more contaminants to remove because the municipal water sources are checked often if they have high lead contents the water sources are often discontinued from use and at the very least will have equipment and filtration put into place to meet the EPA guidelines.  If you had your water tested monthly and found that you have a high lead content in your water even moving your well may not solve the problem and will likely be a very costly undertaking so filtration would be your only safe option for drinking and cooking water let alone for your aquarium.  So just because you do not have the disinfectants to worry about don’t think that you are 100% safe for use.



Now a little talk about the systems and their makeup as well as use.


I am by no means an expert on water quality or the choice of best filters and I am going to do my best on refraining from one supplier over another as there are lots of great ones out there.  I bought my current system from Marc Levenson who has his own salt water tank and also sells a few other items.  His tank at the time of my purchase had just sprung a leak and I felt good knowing my purchase would go to help him get a new build going.  Many times I actually make purchases from companies that I have an advertising relationship with because getting 1% to 5% back on purchases makes the hobby a bit less costly and you will have likely seen the advertisements throughout this article and elsewhere on my site.  And if you like one of my articles or enjoy dropping by to see a few things and want to support the site then click on a link and make a purchase.  I so far have had great service from all of my advertisers and would not put them there if I didn’t trust them.


Now my system is a 5 stage RO/DI system.  The first stage is a 1 micron poly sediment filter to remove larger pieces of rust and sediment from the incoming water.  The second stage is currently a premium grade granular activated carbon filter that is refillable and reusable to help make sure that all of the chloramines are removed.  The third stage is a “chloramine master” carbon block to also help remove the chloramines and act as a filter to catch any stray particles that may come from the previous stage.  The fourth stage is the Reverse Osmosis membrane which really is the heart of the system.  It is a membrane that has microscopic holes in it that basically only allows the tiny single water molecules through and forcing anything large to be forced out of the system and down the drain.


At this point in the system a couple of things can happen.  First of all it can run into a 2.5 gallon dispenser that I fill from time to time and keep in my fridge for cold drinking water.  It can also go through a 20 ft line to my icemaker for super clean ice.  The water can also flow through the DI resin to come out as ultra pure tank water that I fill into 5 gallon jugs for auto top off water or on a rare occasion water for a water change.


The DI Resin is what is called a mixed bed color changing resin.  It is actually two resins mixed together with a chemical that changes from a dark brown to an orange color as the resin expires.  This is helpful so that you can visually see when the resin will need to be changed.


I would like to make a couple changes to the system as it sits now.  First of all there are higher grade DI resins that can be used but they are non color changing so what I am thinking about doing is running a two stage DI with the first stage being the higher grade non color changing resin and the second the color changing kind.  This way when the first DI stage fully expires I can see the color change begin to show and change the first DI stage cartridge with no major loss in quality and no need to do water checks every time I make water to see if the resin is going bad.  I also want to add one or two more stages before the RO membrane.  I would like to filter the water a bit differently than I can now.


The first stage I would like to be a filter of around 10 microns to catch any large sediment but not reduce the water pressure too much.  Then two stages of granular activated carbon followed by a carbon block to guarantee that all chloramines are removed and finally a 1 micron filter to remove any leftover sediment from the system.  I would also enjoy having a dual membrane system so that less water is wasted down the drain from the RO process but I also realize that would also likely require a booster pump which for my current water usage may not be worthwhile as I currently use around 200 gallons per month but when I get my planned 55 gallon tank online it may be more useful.


So the system from start to finish would be something like this:


  1. 10 micron filter
  2. Granular activated carbon
  3. Granular activated carbon
  4. Carbon block
  5. 1 micron filter
  6. RO Membranes
  7. Split to icemaker, drinking water or DI resin
  8. Semiconductor grade mixed bed DI – non color change
  9. Nuclear grade color change mixed bed DI


This would end up being a total of 9 steps or 8 stages and with a second membrane I would have about a 50/50 rejected water vs pure water rate.  It is also my opinion and way of doing things, sometimes I like to do things with as much overkill as possible to reduce possible problems in the future.  My system as it is has had very few problems so far but sometimes leaves me questioning if I can make it better.  I also love the idea of having refillable cartridges that I can use vs a throw away.  I am not a tree hugger but also know that some resources are limited and plastics are made from oil.  If we quit throwing plastics away more can be diverted for other uses and I hate the hit my wallet takes at the fuel pump right now and feel even worse for a buddy of mine with a big diesel pickup.


Now if you are in a locale that highly frowns upon wasting water like Reno, NV where a lot of water is actually reclaimed for use you may wonder what you can do with the rejected water from the RO membrane.  Well I have thought much the same way and though right now I live in an apartment if I had a house of my own which I hope to do in the not too distant future I would likely get a fairly large storage tank and then use the water for irrigation, basically watering the lawn and plants.  You could even take this one step farther and divert water from bathroom sinks to a tank as well as most of the water that comes from those sources is basically only a bit soapy and with most soap having phosphates in them would act as fertilizer for your lawn.  When your neighbors are unable to water their lawn due to rationing during a drought you would legally be able to water your lawn due to the fact of using reclaimed water.  It may take a bit to explain to the authorities when the neighbors call complaining but is better for the environment than just dumping treated water on the lawn and ground or running water needlessly down the drain.  Or if you do not have a large water requirement you can run the waste water into your washing machine you can make about 10 gallons of pure water at a time this way before using the waste water as most top load washing machines will hold about 40 gallons of water.


Another thing to think about when you have an RO system is that the longer the system runs the more efficient it gets.  Large RO system installations will rarely be turned off because the dissolved solids will work their way through the membrane when it is not under pressure.  You will not want to make a gallon of DI water at a time because the initial startup of water from the RO membrane will have higher TDS than when it has run for a bit.  When using my system I will generally make about 2 gallons of regular RO water for drinking water and then about 10 gallons plus of DI water for aquarium use.  The reason this is done is to allow any high TDS water to run out of the membrane and allow the system to really process some pure water before sending it to the DI resin.


You may also be asking why you can’t just run the water through the DI resin and skip all the other equipment, well by removing the majority of the TDS from the water it will allow the DI resin to last much longer.  With a refill package of resin costing about 13.00 though it is not extremely expensive it would get costly if you continually run only the resin as a cartridge would probably only make a fraction of the water that can be made by running highly filtered water through the resin as a polishing stage for ultra pure water.


Well hopefully this information helps a few people out because if you were like me when I first got started in this hobby I didn’t know much of what to do but I knew a long time ago that I wanted to have a saltwater tank. 


Thanks for stopping by,




On a side note even without a TDS meter you can check the purity of water.


Tap water will readily freeze at exactly 32 degrees Fahrenheit, but ultra pure water will actually get much colder and still not freeze.  So get two plastic bottles one filled with tap water and one rinsed and filled with DI water or distilled water and put them both in the freezer as straight up as possible.  Check on them from time to time and when you notice the tap water bottle is frozen pull the other bottle out of the freezer very gently and then while holding the bottle with middling force slam it onto the counter.  The bottle of water should rapidly begin to freeze, this happens because the water has so few impurities in it that the ice crystals have a hard time forming.  When the bottle is slammed it creates a good environment for the crystals to begin to form and this happens very quickly because the water is already below freezing.  This can be useful because if you want to check the purity of water you can perform the same experiment, if the water in the second bottle freezes at the same time as tap water then it is not really pure and should not be used in your aquarium.  A TDS meter is a much better way of checking this but in a pinch this will work, but spend the 25.00 and get a handheld meter.


Please do not reuse this article or any information included without specific permission.