Nitrate Level Is 100ppm And Don't Know Why

The first thing to say is that our nitrate testers aren't the most accurate. You need expensive lab equipment to get really accurate results. And if you are using a liquid reagent tester, one of the bottles (varies between makes) needs to be shaken extremely well or the results will be even more inaccurate.

But having said that, what is the nitrate level in your tap water? The highest allowed level in drinking water in the UK is 50ppm. If your tester shows nitrate above that I'd suspect either something wrong with the tester or your technique.

If your tap nitrate is below 50ppm, something is making your tank nitrate high. The usual suspects are too many fish; overfeeding and not enough/big enough water changes. The usual guideline is that tank water should never be allowed to get to the tap level plus 20.

Have you entered your tank in the Community Creator? What stocking level does it say you have? If it is over 100%, you need to remove some fish.
How much do you feed? Is there ever uneaten food on the bottom of the tank?
How often do you normally do a water change, and what percentage do you change?


If the tap level is well below 50ppm, I would do another very big water change and test again after 30mins (to allow the new water to mix thoroughly). If it's still higher than the tap level, do another large water change next day and check again after half an hour. Once you get the tank nitrate as low as the tap nitrate, keep it there!


If you have tap nitrate at or above the legal limit, you may need to start using RO water to get it down. There is evidence coming out that tank nitrates should never exceed 40ppm, with sensitive fish needing under 20ppm.

Your nitrite should really be zero. Have you added any new fish very recently? If you have the slight nitrite could be the bacteria catching up.
Or done anything to the filter eg changed media? Or even disturbed the substrate by putting in a new plant?

If your tank nitrate is so much higher than your tap nitrate, something has caused it to get that high. Do you add a nitrate containing plant fertliser? If not, the nitrate must be coming from the fish via ammonia and nitrite.
If it's not from plant fertiliser, try doing a few big water changes quite close together which should get the tank nitrate down to somewhere near the tap level. I have a 180 litre tank so I do appreciate that's not a terribly easy task. Once you get it nice and low, test the nitrate every couple of days to see how high it gets in a week. At the end of that week, it should be no higher than tap plus 20.
At 88% stocked, I would change more like 30 to 40% a week. It could be that 20% changes has allowed the nitrate level to creep up bit by bit over several months.

Ahhh, changing the filter media could account for the slight blip in nitrite. You probably also had one in ammonia but missed it.

Ignore what the filter instructions say about changing media. The only thing that does need replacing regularly is filter wool, if your filter uses that. It doesn't wash well, and isn't a very good home for bacteria. Sponges and ceramic media will last for years, just wash them in old tank water. Sponges need replacing when they develop holes or won't go back to shape after squeezing; ceramic media need replacing when they start to crumble.
When they do need replacing don't replace them all at once - it was good that you only changed 50%.


If you do a few bigger than usual water changes a bit more often than usual you'll get the nitrate down faster. Once it's down you can go back to slightly larger, weekly water changes.