Dealing with high nitrates in aquariums

What should I do if I have high nitrates in my aquarium?

The first thing to do if you have high nitrates in your aquarium is to find the cause. You can deal with most causes of high nitrates by simple changes to tank husbandry. Is your tank overstocked? Are you skimping on partial water changes? Do you skip cleaning waste from the gravel? Are you over-feeding ? For all of these, the solution to the problem is easily within your control.

How can I reduce nitrate levels in the tank if my water supply already has high nitrates?

High nitrates in tap water is a common problem in the UK. You can reduce high nitrates in tap water either by taking nitrates out of the water before it goes into the tank or you can make changes to the tank and filters to allow some nitrate reduction to occur inside the tank.

Mixing tap water with RO water is the straightforward option, especially for those who also need to reduce water hardness. It’s easy to calculate the reduction given by any mix ratio as the concentration of nitrates reduces proportionally: A mix of half tap water and half RO water will halve the nitrate level. You can buy RO water from many aquatics shops or you can install your own RO filter. RO water must be remineralised for use in a fish tank as it has none of the minerals essential to fish health nor buffering capacity and so has unstable pH. If you intend using RO please read these articles: What is Reverse Osmosis (RO) Water? – Why, when and how to use it and RO water in your aquarium – changing pH, gH, and kH
Another alternative is to use a nitrate reducing filter. This will remove nitrates from the water, leaving other aspects of the water chemistry untouched. These can either be permanently plumbed into your water supply, with their own tap or used in a temporary arrangement with a tap adapter as and when needed.

A third option is to fill a container with tap water a couple of days before a water change and run an internal filter filled with nitrate reducing media in the container. A heater can also be added if desired, to bring the water to the correct temperature for your tank.

Final Comments

Please remember that reducing nitrates by any of these methods can be useful where there is a real problem, but should never be used as a substitute for correct stocking levels, regular water changes, and proper tank maintenance.

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