Reducing Nitrate via Water Change

The amount of water changed in an established aquarium is basically the only way to reduce nitrate levels via water change. If you aren't getting results in reducing your nitrate to safe levels through water changes than you either aren't changing enough water and/or the water you're adding back into your tank is high in nitrate.

If you have a tank that has an unsafe nitrate level at say 80ppm (Parts Per Million) and your tap water has a safe nitrate level at say 20ppm than doing a 25% water change will not alter the nitrate level (it may reduce other things in the tank but nitrate won't be one of them). Even if you did numerous 25% water changes under these circumstances your nitrate would still go unchanged. Now if your tap water had a 0ppm nitrate level than any % of water changed would reduce these nitrate levels.
Given the above stats as before if you did a 50% water change than you would actually reduce the nitrate to 60ppm. If you continued doing these 50% water changes than over time the lowest your nitrate level could get would be 40ppm (which is a safe nitrate level). Keep in mind that the lowest your nitrate level can reach depends on the water you are adding back into the tank and in this case the only way to reach 20ppm would be to perform a 100% water change because that is the same nitrate level as the tap water. Another interesting thing to note is lets say you did a 75% water change and your nitrate is now at 40ppm and the following day you want to do a 50% water change to further decrease the nitrate. Doing that would not reduce the nitrate, it would actually raise the nitrate to 60ppm if the tap water is still at 20ppm. So it is possible to raise your nitrate level even if you are performing regular water changes.
It is important to know that nitrate levels generally increase overtime so, an aquarium can reach unhealthy nitrate levels (which generally start at 80ppm) which can cause "Nitrate Poisoning" in aquarium inhabitants. A gradual increase in nitrate may not appear to harm fish over time but it can harm/kill newly added fish that have not been exposed to this level of nitrate. One symptom of nitrate poisoning may be "Swim Bladder Disease" which affects equilibrium in fish. By measuring nitrate levels (I use Tetra EasyStrips to test my water) weekly it can help you figure out how much water you should be changing and possibly how often it needs to be changed.

Big water changes can have ill effects on aquarium inhabitants that have gone without regular water changes so, it is often best to start small and work your way to doing bigger water changes, as again you may not be changing nitrate levels at first but you are more than likely altering other water parameters (such as reducing any aquarium additives). By drastically reducing nitrate in an aquarium you risk putting the aquariums inhabitants through things like "Nitrate Shock" and some symptoms of this are listlessness, loss of equilibrium, and fish laying at the bottom of the tank.

Aside from water changes some other ways to help keep nitrate levels low is to not overfeed aquarium inhabitants and use nitrate removing filter media (which does not mean water changes should not be performed). There are also water treatments available which may help to reduce nitrate but some of these may also contribute more to polluting a tank causing nitrate levels to rise in the long run if water changes are not continued so, I don't particularly see a need in purchasing those unless the new water you're adding to an aquarium is very high in nitrate as well (such as water with a nitrate level over 40ppm).

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