The level of nitrates tolerated without ill-effect varies from species to species, but as a general rule-of-thumb the level in the tank should be kept as low as possible: always below 40ppm and preferably below 20ppm. Some species have a particularly low threshold for nitrates – Fancy Goldfish need levels below 20ppm as higher amounts can cause buoyancy problems which are frequently mistaken for Swim Bladder Disease. Where fish have low nitrate tolerance for such species will usually indicate this.

Water should be tested weekly using an aquarium nitrate test to monitor the level so you can be ready to take appropriate action when levels rise.

How do nitrates get into the water?

Nitrates are the end product of the Nitrogen Cycle. Any aquarium with life in it will accumulate nitrates as a natural consequence of the filtering process. Standard filtration does not deal with nitrates and they must be removed by regular partial water changes. A typical freshwater aquarium with the correct amount of stock, efficient filtration and correct feeding will need a water change of around 25% every week to keep nitrate levels in check and to replenish essential minerals and trace elements in the water that have been consumed by the tank’s occupants and plants.

Nitrates are usually present in the tap water used to fill the tank.  The level varies with the source of the tap water, but in the UK must be no more than 50ppm by law.  Most UK water suppliers provide information by postcode on various aspects of water quality for the locality, including the average nitrate level, but because this can still vary it is advisable to use your test kit to establish the nitrate level in your own water supply.

What should I do if nitrates in my tank are too high?

The first thing to do when you realise you have a high nitrate level in your aquarium is to establish the cause.  Many of the common causes can be remedied simply by changes to tank husbandry.  Is your tank overstocked? Are you skimping on your partial water changes?  Are you forgetting to clean trapped waste from the gravel? Are you feeding too much? If the answer to any of those questions is “Yes”, the solution to the problem is easily within your control.

If your tank is correctly stocked, not overfed and is properly maintained by weekly gravel vacuuming and 25% water changes then the problem may be that nitrates in your water supply are high. The level of nitrates in the tank can never realistically be lower than that of the water you put in, so if the water from your tap already has a high level it makes life a little more difficult.

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

There are a number of ways to approach this, but all involve either taking nitrates out of the water before it ever goes into the tank, or changes to the tank and filters to allow some nitrate reduction to occur inside the tank.

Reducing Nitrates in the tank itself

This approach is the simplest, especially where tap levels are higher than you’d like, but still a reasonable amount less than the 40ppm maximum recommended.  Keeping to a minimal stock level will obviously help, as will taking care not to overfeed, removing uneaten food daily and increasing water changes to twice a week instead of once.

Adding more live plants can produce a quite respectable reduction as the plants will consume some of the ammonia produced by the fish before it is converted to nitrate. Floating plants such as Duckweed are particularly good for this.

Nitrate absorbing sponges such as those made by Juwel can be used in your filter, but must be regularly replaced as they become exhausted. API Nitrazorb Pouches can be placed in your filter in a similar manner, but have the advantage that they can be recharged when exhausted by soaking in salt water.

Products such as Seachem DeNitrate and Seachem Matrix can be used inside filter canisters as part of the biological filtration mechanism and have the advantage that they do not need to be regularly replaced or recharged. Seachem DeNitrate is only effective in low flow filters running at less than 200 litres per hour so it may be advisable to add a secondary low powered filter filled with this specifically for nitrate reduction. Matrix can be used in higher powered filters. 

Liquid additives such as Tetra Nitrate Minus contain small beads impregnated with nitrate-eating bacteria. Because these bacteria are typically anaerobic in their action, their life is limited when introduced in this way and the treatment needs to be repeated weekly to maintain satisfactory nitrate levels.  It is a very simple treatment to use as it is added directly into the aquarium, but its disadvantage is the ongoing cost of regular treatment.

Reducing Nitrates in Tap Water

Where tap water nitrate levels are unacceptably high there are a number of ways to reduce the concentration before adding the water to your tank.

Mixing tap water with RO water may be a very 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.  E.g. a mix of half tap water and half RO water will halve the nitrate level. RO water can be purchased from many LFSs for quite a small cost, or you can install your own RO filter to produce RO water. RO water should not be used “neat” as it has none of the minerals essential to fish health and has no buffering capacity and so has unstable pH.

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 adaptor 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.