Reverse Osmosis for Aquariums?

Is Reverse Osmosis Water Necessary for Aquariums?

Over the years I have often come across quite some interesting questions posted by aquarist about experimenting and doing different things related to how they can manage their aquarium hobby differently. One of it worth mentioning here is from a guy who owns a JBJ aquarium who queried about the possibility on using reverse osmosis water for his saltwater fish tank. Here’s my take and views on this.


RO systems are mainly used in households to remove impurities and improve the quality of the drinking water. And because of its availability, people have arrived on the idea about using it for their aquariums as well and since it is there, then why not uses it and tries it out? But before I move on further, for your knowledge, water coming from the tap contains a certain amount of dissolved solids in the form of salts, phosphates, nitrates with varying degree and concentration of chloramines and also fluorides and that depends on the location on where the water sample is taken from. Basically what you are trying to add to your fish tank is the water free from all those solids, practically pure with no hardness level while the pH of RO water is generally lower compared to normal fresh tap water.

Based on my experience there is no harm in using RO water to change your aquarium water but at the same time, it will also not change anything at all if you choose to use normal tap water instead. And that is provided that you properly treat the tap water using water conditioners or leave it overnight so that the residual solids that settle at the bottom are removed and the harmful chemicals in it are already degraded. As with the case of the JBJ aquarium owner, his question about exploring the possibility about using RO water does not seem to make any difference at all and that is especially true when it comes to saltwater tank. Let me put it this way, the process of changing the water in a saltwater aquarium is a very delicate process. The main thing which you will need to control is not to disrupt the salinity of the tank and if you are withdrawing the water out of the aquarium, adding fresh water will lower the salinity level. Thus, whether you are using reverse osmosis or normal tap water, no matter which one that you choose to use, you will have to add the natural sea salt mix in order not to disrupt the salinity level. Most tap waters contain very little or very low amount of these impurities and since the water is going to become a saltwater mix anyway, whether you are adding a reverse osmosis water or otherwise will have little effect. I hope I have made my point clear at this stage.

On the other hand, if let’s say, the water level in the tank drops because of natural evaporation, and since salt will still continue to remain in the system, it would be safe to just top up freshwater going into the tank without bringing any harm to your reef or even your fish. A word of caution though, as you might want to slowly add in the water in order to prevent sudden shock to your pet fish. But what if it comes to freshwater aquarium? So far, I haven’t really tried doing that for my freshwater tank since the cost of servicing and replacing the RO membrane can be costly but however, even if I can afford it, I wouldn’t go ahead with the plan. However, there are aquarists whom I know decide to use and mix a certain percentage of the reverse osmosis water to tap water before putting it into the aquarium so that they can adjust/control the water hardness and pH accordingly. But for me, there are few reasons on why I’ve decided not to.

1.First of all, once you have decided to use RO water, you should make it a point to constantly use it every time you top up or change the water as your pet fish will have already adjusted to the water condition. It would be unwise to constantly use RO and suddenly switched to normal tap supply as this will put unnecessary stress on your fish as again they will take time to readjust.



2.RO water also lacks no mineral content and other trace minerals that the fish needs and although there is no conclusive research or evidence that points to the harmful effect that this condition brings, I will still try to avoid it. If you have cichlids tank (either Malawi or Lake Victoria), that would turn into bigger problem as the species would require hard water for survival.


3.And finally as what I’ve mentioned earlier, cost is still an issue you might want to consider. A 100-gallon aquarium with 25% water change will still require a huge amount of investment to convert it to RO water and imagine the amount of money you will need to spend for this purpose.

Nice write up. I agree!

Great article I decided to stay with plain and simple. It seems like everyone that tries to perfect PH/Hardness ect ends up having problems when they forget to add whatever to the mix.


>>>> On a side note, I wanted to ask if you know who wrote these articles? Most of them seem to be coming from http://www.allabout-aquariumfish.com/2011/06/is-reverse-osmosis-water-necessary-for.html word for word but no credit given here. No author is listed for each page but the owner of the site runs the blogs so I figured they did it. Sock Yee and some guest writers as well. Just wondering.

Yes , My fault I forgot to post the link where it came from. I try to do that to them all. I did get this one from all about aquarium fish. Lot's of good info everywhere if you browse around.

in marine tanks, the 1st basic consideration is water. Without it, nothing in your tank would live for long. Compared to freshwater tanks, marine tanks need excellent water quality to maintain success.
One of the things about water is you don't know what is in it until you test it for yourself. Fluoride, Chlorine, Chloramines, Nitrates, Phosphates, and even metals none of which can be in your marine tank safely. Many of these compounds will create nuisance algae and may even lead to premature death of livestock. Buying water from stores might be safe, but you don't really know when they changed their filters last. Look at the LFS display tanks. Are they algae free, or is there algae outbreaks in their tanks? If you see a lot of algae, their water probably isn't best to use. If you choose to use tap water because it saves you money, you'll need to add some type of de chlorinator to protect your livestock from chlorine. then think how much is your time worth, when you have to spend hours and hours battling green hair algae or worse?
Getting an RO/DI unit of your own is the best/smartest decision you will ever make. The up front cost of a unit can vary depending on where you buy it. There are many styles with a variety of options, and they can range from about $100 to $300.
The benefit of having a unit in your home is that you can make pure water as you need it. Installation takes about 10 minutes. You won't have to carry heavy containers of water 5 gallon buckets out of the store, load it into your car, then unload it at your house and carry it to your tank. Another benefit is you know exactly when your filters were changed You can even test your unit to make sure your water is safe. A TDS meter will measure the water quality zero TDS is the goal, I have an inline TDS meter that cost me about 25 bucks. The DI cartridge changes colors from black to brown indicating when it has been consumed.Maintenance is easy. Change the filters sediment and carbon every 6 months and the DI once a year. The RO membrane should be good for 3 to 5 years. I keep a Reef journal of the dates I changed my filters, bulbs etc..I also store 30 gallons of RO/DI at all times not only did it come in handy for the tanks but we have power outages here and it makes some really tasty coffee:)
If you add up the cost of the water you are purchasing or the chemicals you buy to de chlorinate your tap water), you can see that in very little time an RO/DI unit will pay for itself. And your marine tank will look the better and be healthier for it.
BTW I top off 4 gallons every morning in the summer months and I dump it right into the sump.LOL I have an ATO in the closet but I can't bring myself to trust it:)

Sorry, I'm not experienced in SW setups. Is it another common reason why some people would rather have RO systems for SW is because of copper plumbing in their homes? Doesn't the copper have an impact in the tap water? Just wondering.

I had once thought about an RO system when I was raising discus adolescents to breed. I didn't have the money to get it, though, and was still able to raise huge discus to maturity using my tap water. On the other hand, I had to deal with pH, buffering, resin bags to reduce hardness. There are upsides and downsides to having an RO system.

Water quality =

Go Ro/Di...

Go to some ghetto gas station and get an electric 'scale'..(be careful mind you)
always use weight, not 'measurments'...weight doesn't lie..

Freshwater fellows.. Get "equilibrium" and some Seachem alk buffer.
I also use 1tbs/5gal sea salt... (you use you're judgement I dont know your fish)

Here where I am, the saltwater place makes its own Ro/Di and adds an
"all in one" mixture. I can't be accountable for it but there fish are perfect...
Next time I'm there I'll get the brand name..

Tap is garbage... chemicals, upon chemicals, and more to fix the chemical problem.....

Beware the well water, the metal is bad, and usually it's in a 'farm area'.. and the fertilizers
will run off and burn it all up... overnight it seems, this happened to a friend of mine....
He told me so I tell you....Anyways, moral of the story, if you dont 'make' your water
you have no idea what's in it... it's worth all the time/money.... - peace