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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.

aquarium reef systemRO 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.

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