RO water and its basics

Hi there,

@Zoom, you requested I start a thread on this subject, so here goes. Please note that this is by no means an attempt to scientifically explain all in infinite detail. See it rather as a beginner guide into basic water chemistry.

I will start by explaining a few things we as aquarists refer to often, and which is directly influenced when using RO water.

What exactly is RO water and why the hype?

RO water is water, from any source that has passed through a Reverse Osmosis filter. Different filters exist which ultimately give different quality end product. The basic purpose of RO water filtration is to clean and strip water from all its dissolved solids, finally leaving you with as close to 100% pure water as you can economically get.
Certain filters are called 3 stage, others 5 stage and even 6 and 7 stage RO filters are available. The latter will run the water past a remineralising stone and then effectively replace the minerals and Salts to a predetermined level. For aquarium use most people opt for a 5 stage filter, as commercial remineralising blocks do not always provide the desired end product.

Basically an RO filter consist of
1) either one or two sediment filter stages removing large floating particles.
2) An activated carbon stage removing chlorine, chloramine and some toxins
3) a Reverse osmosis semi permeable membrane allowing ONLY water to pass
4) a water polisher improving taste, odour etc.
5) Remineralising stage (only certain models)

When water pass through an RO filter, only a percentage of the water that went in come out as useable RO water. The rest, carrying the 'nasties' removed by the filter is routed as waste water. The percentage usable water varies depending on the quality of the filter, as well the "sieve" size of the membrane, but averages to between 20 to 30% usable RO water per volume. This means that to get 3 liters of RO water, 10 liters of water has to enter and pass the filtering process.
End product = 98 to 99.9% pure water.

This water should ideally, in a well functioning high quality filter end up with a TDS (Total dissolved solids) value of 0 (zero)

That brings us to the next thing - TDS, total dissolved solids. TDS is an expression of the solution of minerals and salts in water. It gives a value in 'parts per million' (ppm) of all organic as well as inorganic substances contained in the water. The cleaner the water is of minerals, salts and other elements, the lower the TDS will be, and vice versa.

How is tds measured? The most accurate way would be to take a volume of water, allow it to evaporate and then weigh the remaining sediment. This is not feasible for everyone, and for that reason a TDS or Electrical conductivity meter is used.
Effectively the same thing, the two different meters measure in the same manner the TDS, but express the result in different values.

E/C Meters measure the ability of water to conduct electricity. Absolute pure water is a weak conductor of electricity, so conductivity is directly related to the amount of minerals and salts dissolved in the water. These salts dissolve into positively and negatively charged ions which conduct electricity. The higher the TDS, the more charged ions and in turn the better the conductivity potential of the water. It is thus the ions from the dissolved solids in water that creates the ability for water to conduct electricity. The measurement is then done and expressed as the amount of electricity conducted per cm of water.
TDS is expressed in parts per million (ppm), and Electrical conductivity in micro/milli Siemens (Siemens being a value added to ohms)
Electrical conductivity is influenced by temperature, but most good meters will give a value as if at 25'C
Distilled water has an E/C of 0 (zero), and RO water between 0 and 20milli/S

When using a meter that gives conductivity results in milli Siemens, a formula is used to convert this to TDS. This formula varies somewhat, but on average the value in E/C can be multiplied by 0.67 which will then give you the TDS value in ppm.

GH -General hardness.
Primarily this gives an indication of the calcium and magnesium ions present in water. Gh does not affect ph of water, but hard water is generally alkaline due to the interaction between Gh and Kh.
Gh is normally measured and expressed as 'degrees'. The higher the value, the harder the water as it effectively means a high volume of calcium/magnesium

Kh- Carbonate hardness
Measure of the alkalinity of water due to the solution of carbonate and bicarbonate ions. These ions act as a buffer to stabilize ph levels in the aquarium. The higher the Kh value, the more buffering exist, and the harder it would be to alter ph levels. At very low Kh levels on the other hand, the nitric acid that builds up in the aquarium due to the nitrification process will cause the ph to drop. If the buffering capacity of the water is very weak, the ph can fall far, and in a matter of hours. This normally result in the death of fish.

Ph - ph is an indication of the acidity/alkalinity of water. The scale, 0 to 14 with 7 as neutral indicates the amount of hydrogen ions contained in the water. The more hydrogen ions, the more acidic the water is. An important fact about the ph scale is that it is calculated in factors of 10. This means that for every 1' drop in ph, the hydrogen ions have increased tenfold. This means that a drop from 7 to 5 would mean the hydrogen ions in the water increased 100 times, so the water has become 100 times more acidic. This is why a stable ph in well buffered water is such an important factor.

Now, to put all this together.
RO water, with a TDS level of 0 as previously indicated, also has a Gh and Kh value of 0, and a ph very close to neutral. (pure water has a ph of 7)
Using this water indiscriminently in aquaria will result, again due to nitric acid, in a severe and unforseen ph crash. Furthermore, RO water as such contains no minerals for fish to absorb, and this can only be corrected with a very good diet. Why bother?

Most fish available for aquarium's today have been bred and raised in water similar to our tap water, and therefore will do fine without RO water.

It is however possible to use RO water in aquaria. Reconstituting salts exist to effectively "reload" the RO water to the desired levels. To increase Gh epsom salt can be used, and to increase the Kh (buffering capacity) baking soda can be used. Reconstituted RO water, done correctly is safe as no unwanted goodies will be in the water without your knowledge. The same can not be said of tap water.

However, I still feel that for most fish keeping tap water is fine, and for certain fish a RO/tap water mix will suffice. For breeding certain species this can also be done.

To summarize-Tap water in most parts of SA tend to be hard water with an alkaline ph (Gh+8dh, Kh+8dh, ph 7.6 TDS 200 to 350ppm) RO water (Gh0, kh0, ph+-7, TDS 0ppm) can be used to soften this water but it should be done wisely and with knowledge and understanding of the vital role Gh and Kh has on water parameters.

Just for interest sake, I'd like to also point to the use of E/C Meters in aquarium maintenance.

Due to evaporating water leaving behind minerals and salts, and also an increase in nitrates, TDS in aquaria will always rise in the aquarium. It is for this reason that we do water changes. An E/C Meter can thus be used to establish at which stage water changes are done, and it will also be used when mixing RO/TAP water to desired levels or reconstituting water by means of salts and trace elements.
Fluval recently launched their G-series cannister filters with built in E/C meter.

For referance

TDS for general aquarium fish and maintenance 75 to 200ppm

For African Rift valley Cichlids 200-350ppm

For Discus and other soft water fish 35 -75ppm

Many people think that Discus originate from the cleanest water with the lowest TDS/EC values. Not entirely true...
Earlier I said that Distilled water has a E/C of 0milliS, and RO water anywhere between 0 and 20milliS depending on grade of filter.

Heckel Discus, renowned for their preferance of clean water are found in areas with a conductivity of between 10 and 30milli Siemens.
However, cleaner than even that, with a conductivity sometimes below 10 are the waters in which you can find Cardinal Tetra's and German Blue Rams. No wonder many of struggle to keep them alive...

I hope this thread has been of some help. Please add or correct where I have gone wrong. Thanks for reading

Rgds,

Marco

 

Thank you for writing and posting this Marco. Another, excellent, informative and well written article. Have you ever thought of writing a book?

Just one point to add, is that if using R/O it may be necessary to oxygenate it before use, as the R/O filter removes oxygen. I usually use an airline in a barrel with the R/O in it for a few minutes before use.

Mark

 

@Marco - Dude... Brilliant thread, something I've been meaning to speak about for some time now.

I have a question though... We all add tap water to top up evaporation. Surely adding tap water every couple of days to top up the water is making even more problems in your tank? You are essentially changing your waters parameters by adding tap water gradually because you are adding more elements that are already present in your tank. So you could make your water harder over the period of a few weeks/months? Not even a 50% water change will correct this, as you keep adding extra elements when you top up... It's only H20 that evaporates, not the left over elements like calcium, etc...

Could this not lead to potential imbalances in your fish tank? Surely it would? Perhaps the reason why algae starts to increase as the tank gets older?

So now, surely if we add RO water as the topup water, you are pretty much guaranteed to not be changing your fish tank waters chemistry? Thus keeping your water parameters as stable as possible?

It's something that I picked up from the marine side, but surely we could apply this to fresh water as well?

 

nice article marco!

in a marine, community or malawi type setup topping up with RO would be ideal as there is not much going on to affect you water chemistry. if you have a sps setup then obviously this does not apply as you are needing to add kalkwasser etc.

in a planted tank especially running co2 and high ligt your plants are stripping the carbonates from the water, so unless you have a pretty good idea of the rate that these are being used then you are better off adding tap water to replace these. having said that adding remineralized RO water would be even better coz you not getting any of the other stuff that comes along with our tap water, but only for small tanks.

as marco said the water wastage from generatin RO is very high, imagine doing a 50% water change on a 400l tank with RO water. you would be wasting the 200l you take out plus the 800l of grey waste water, so a decalitre of water down the drain once a week... that is crazy..

perfect water parameters are like chasing the dragon, you will cause yourself endless hassles trying to achieve perfect when stable is just as good imo. you have no idea the lengths people will go to, to get perfect parameters, you want to lower the ph so you add some hydrochloric acid, this will also decrease your buffers, carbonates and increase your tds. so even though your water may now seem perfect, it is worse than it was before and you are risking a ph crash..

most fish and plants will adjust to your water if it is in a range, and both prefer stable water params to perfect. i see so many posts about fish being stressed after a large water change and crypts melting etc. but instead of making their tanks more like the tap water, they are trying to adjust the water to perfect and end up inducing tds shock and gh and kh shocks, because the ph is out sligtly. fish and plants are much more tolerant to changes in temperature and ph than they are to gh,kh and tds...

i use RO water in some of my tanks, but they are small and it is because the inhabitants are extremely sensitive and wont breed otherwise. if money and resources were not taken into account then i would use RO and co2 in all my tanks to achieve the parameters i wanted, but tap water works just fine for me most of the time. i am however going to get a uv light to kill off the cyano that our water board can't seem to get rid of, that i keep introducing with every water change...

 

Hi Guys

Thanks guys, glad you found it helpful

@Sean J - What should also just be kept in mind is that with top-up water, we never remove any minerals as you have stated. What also stays behind, apart from minerals and salts, is the nitrates. So doing top-ups only will result in ever increasing nitrate levels, as these are effectively never removed.

I hardly ever do top-up's. My water changes are done so frequently that I do not ever reach a stage where I need a top up. This way I easily maintain a TDS level of below 150ppm in my Discus tanks at most times, using Tap water only.

@wearsbunnyslippers - very good post - all you have said is very applicable and important. Stability is THE most important thing in fishkeeping, and that means establishing routine in your life to ensure that stability remain present in the tank water at all times.

I always find that the most successfull aquarists are not the ones with the most knowledge and/or understanding - its the ones with the most discipline when it comes to the routine maintenance needed to ensure the best, not perfect as you have also said, environment for their fish.

Rgds

Marco

 

Hi @Reedfish

Actually, to my knowledge the 'water polisher' is just some coconut shells. It leeches something in the water which improves taste. For aquatic use it is thus not needed.

The ion exchange unit you refer to is now also available here. It uses deionising beads or resin to remove certain minerals by means of electrolyte binding and exchange. I am not up to date on exactly how they perform, but from what I understand it works the same way peat moss does. Peat binds carbonates and release humic acid, effectively lowering kh and ph. Resins do the same with the elements they are charged with. Its a matter of negative and positive ions attracting their opposite.

I think the only benefit this will have is lowering of ph. A good RO unit should give u pristine water, and thus no further should be nescessary. It might also just be a case of people trying to make an extra buck out of unnescessary sales.

I will look into it though and let you know what I found.

Rgds

Marco

 

Hi @Reedfish

Actually, to my knowledge the 'water polisher' is just some coconut shells. It leeches something in the water which improves taste. For aquatic use it is thus not needed.

The ion exchange unit you refer to is now also available here. It uses deionising beads or resin to remove certain minerals by means of electrolyte binding and exchange. I am not up to date on exactly how they perform, but from what I understand it works the same way peat moss does. Peat binds carbonates and release humic acid, effectively lowering kh and ph. Resins do the same with the elements they are charged with. Its a matter of negative and positive ions attracting their opposite.

I think the only benefit this will have is lowering of ph. A good RO unit should give u pristine water, and thus no further should be nescessary. It might also just be a case of people trying to make an extra buck out of unnescessary sales.

I will look into it though and let you know what I found.

Rgds

Marco

 

Hi Marco,
Great article - certainly clarifies a few things for the not-so-brainy people like me.
At risk of being slightly off topic, although I see some RO units also have the polisher added as an optional extra, could you also comment on the use of Peat within the fish tank set-up for discus. What are the real advantages - trace elements / resins leaching?
What are the risks I should be careful of?

I normally do 33:66 tap:RO water ratio which seems to work well for me. However on my last water change yesterday I took the plunge and placed a small bag of peat as a final polisher in my aquarium filter.
How much of the stuff would you recommend I use? How often should it be replaced? Should this be done prior to adding the new water to the tank or (like I've done) is it OK to let it work in the filter on a full time basis?

 

Hi

@Reedfish -Thanks bud, thats more or less how I understand the DI unit as well. One thing I do like about it is that it tends to lower the ph slightly, to about 6.5, which RO doesnt do with my tap water.
Please, with regards to peat, I am not suggesting it is as "strong" as resins, merely that it functions on the same principle, thus softening water. Thanks for that feedback Reedfish

@Ramrezi - Peat moss has a lowering effect on Kh of water. Kh -Carbonate hardness, is essentially the water's buffering capacity, its ability to absorb acid's in the water without the water's ph being lowered.
Peat moss lowers this by carbonate binding and furthermore releases tannins into the water which will, in time, along with the lowering kh, get your ph to drop. Peat is however unstable and can have no effect for a number of days and then suddenly reduce the kh very quickly, as low as o'dh, with an acid fall soon to follow.
If you are using RO water in the mix you have indicated I would first test Kh to see if it is actually a problem, which I am pretty sure it will not be. A kh below 4'dh in a general tank starts getting risky as the ph can become very unstable at these levels. I would not leave the peat in your filter, simply because this will lower the kh constantly without it being monitored.
I am sure with the 33/66 mix you are presently doing things will be just fine.

Regards

Marco

 

Ok I know this is an very old thread but I'm wondering what happens to the waste water can I use it in my garden by letting the waste run into another container or does the Waste water run directly into the drain. I'm one of those people that try to reuse as much water as possible I never discard my wc water I store it and use it to water my plant.

 

Plants love ro waste water. I run mine into the pool but i make a lot of ro water for my marine system. Dont run it into the drain as it is perfectly usable

 

@Reedfish good to know I would hate to waste so much water but I really want to get a RO system as I found our water parameters aren't nearly as stable as I would like it to be.
@mjmza I also thought about how much plants would love RO waste water with all of those minerals and stuff in it. Plants love aquarium water to and grey water from baths and laundry. Only water I don't really reuse is dishwashing water I'm worried about the amounts of fats present.

 

Ah yes - only real solution to that is MacDonalds for dinner every night ;-)

Man we have some great posts on TASA ... good digging in finding this one.

Later Ferdie

 

Yeah

I don't think so I'm not much of a fan of eating plastic and silicone disguised as food lol.
As I said I love reading so digging around in old posts is something I enjoy to do