Dwarf Neon Rainbow Fish

The thing that stands out from your test results is the nitrate level. 100 ppm is high.
From Seriously Fish

High water quality is essential to the well-being of this species

If your nitrate reading is even approximate, it is too high for dwarf rainbows. Some sites recommend 50% weekly water changes, and with this regime nitrate should never reach 100 ppm. Ideally, nitrate should never get higher than the level in the tap water plus 20.
Causes of high nitrate are too many fish, overfeeding and water changes that are too small/too infrequent. And it is not only nitrate that builds up, lots of other things we can't measure also build up. A high nitrate usually implies high other things too.
I suggest trying a few extra water changes of more than 25% to get the nitrate down to the same as your tap water, then either do 2 x 25% water changes a week, or one 40% change. And monitor your nitrate to make sure it stays as low as possible. The lowest you can get it will be the same as your tap water level unless the tank has a lot of live plants.

Did the fish show any kid of sores? When I was researching fish recently I kept coming across references saying that dwarf rainbows are prone to ulcer like sores nowadays.

You do have to keep nitrite at zero, and also ammonia. Those are the fast killers. Nitrate is a slow killer and some species must have low nitrate. It is now being suggested that nitrate over 20 ppm will shorten the life of all fish, but this can be a problem as the UK allows up to 50 ppm in drinking water.

Nitrate can also be used as an indicator of water quality. There are so many things we can't test for that build up in a tank - things excreted by the fish, things like hormones secreted by the fish. Many years ago I worked in a hospital lab and you'd be surprised how many things we tested urine for. And that was just the clinical chemistry section. Fish excrete similar chemicals. If nitrate is high, those other things will be high too, and some fish are stressed out by these things dissolved in water including rainbowfish.

Nitrate is the most inaccurate of our testers. It could be worth testing your tap water then looking on your water company's website for their water quality report for your area. That should include nitrate and you can compare your test result to theirs and see how similar they are.


Floating plants are the best type for removing nitrate. They are very close to the tank lights and being on the surface they have access to lots of carbon dioxide as well. Both these are needed from strong plant growth. Look at everything from duckweed through Salvinia to water lettuce and water sprite. Some floating plants get quite large across and without knowing just how wide your tank is it is hard to suggest just one species.