How to lower nitrates in a freshwater aquarium

Keep nitrate under 20 to 40 ppm. If your test reads higher, change water first. A 50% change cuts the number about in half today, and the other five fixes below keep it from climbing back.

Nitrate is the end product of the nitrogen cycle. Your filter bacteria turn fish waste and old food into ammonia, then nitrite, then nitrate. Nitrate builds up between water changes in every cycled tank. A high reading means more waste is going in than you are taking out.

1. Do a water change

This is the only fix that works the same day. Change 25 to 50% of the water with a gravel vacuum, add dechlorinator, and match the temperature. The water change guide walks through the bucket routine.

Here is a 20 gallon example. The test reads 40 ppm and your tap reads 0. Take out 10 gallons and refill, and the tank reads about 20 ppm. If the reading was 80 ppm, do 30% now and 30% two days later rather than 75% in one go. Fish that have lived in high nitrate for weeks handle a gradual drop better.

2. Test your tap water

Run the nitrate test on a glass of tap water with the same liquid test kit. Some city supplies and many wells in farm areas read 10 ppm or more straight out of the faucet. If the tap is high, water changes have a floor, and the other fixes matter more.

Shake the second nitrate bottle hard for 30 seconds before you use it, and wait the full time on the chart. A short shake is the most common reason for a reading that is too low.

3. Feed less

Most high nitrate starts at the food lid. Feed what the fish finish in about two minutes, once or twice a day, and skip a day each week. Pull out uneaten wafers and pellets. The feeding guide has amounts by fish type.

4. Vacuum the gravel and rinse the filter gently

Waste that sits in the gravel and the filter keeps breaking down into nitrate. Push the gravel vacuum into the substrate at every change, and work through a different third of the tank each week if it is heavily planted.

Once a month, squeeze the filter sponge in a bucket of old tank water until it stops clouding the bucket. Do not rinse it under the tap. Chlorine kills the bacteria, and you trade a nitrate problem for an ammonia one.

5. Add fast-growing plants

Plants take up nitrogen as they grow. Slow plants like anubias and java fern barely move the number. Floating plants such as Amazon frogbit, red root floaters, and salvinia grow fast and pull nitrate from the water. Hornwort and water sprite work too.

Thin floaters to about half the surface each week and throw the extra out. The nitrate leaves the tank with them. A low-tech planted tank covers light and fertilizer basics.

6. Check your stocking

If you do all of the above and nitrate still climbs past 40 ppm within a week, the tank has more fish than its water and filter can handle. Goldfish are the usual case. They make far more waste than a school of tetras the same size. Rehome a few fish, move to a bigger tank, or plan on two changes a week. The stocking calculator gives a rough check on the load.

What not to do

Skip the 90% change and the full tank strip. A sudden drop in nitrate and a swing in pH and hardness can shock fish that have adjusted to old water. Do not replace all the filter media at once either.

A brand new tank with no nitrate and rising ammonia has a different problem. That is new tank syndrome, and the fix is to finish the cycle.