What was the nitrate reading before you did the water change?

Doing a 50 % water change is always a good idea when you have any kind of  water abnormality or your fish's behavior is different in a bad way (darting around etc..).    So if your nitrate is 40 now, do a 50 % water change today , which should reduce the nitrate to 20 and perform another 50 % change tomorrow also.   You don't want drastic changes in water parameters like nitrate , pH, and water temperature , because it can shock the fish or possibly even kill them.   So performing a 50 % water change twice a month is better than doing doing 90%  water change once a month.

Some causes of nitrate spike:

1:)  Overcrowding = too many fish for the tank size and overfeeding your fish (solution = get appropriate tank size and only feed what your fish can eat in a few minutes)
2:)  A lot of fish poo and uneaten food in the gravel  (solution = vacuum the gravel)
3:)  Dirty filter (solution = clean filter pad/or filter cartridges to remove fish wastes and other organic matter. If the filter pad is in good condition, you can rinse it in old aquarium water and reuse it many times before you need to replace it .)

 Adding plants can also help to control nitrate level . Plants utilize nitrate for growth. You should also perform regular water changes (like 2 times a month)  to help control the nitrate level in your fish tank. The frequency of your water changes is dependent on your fish stocking level and how much you feed them.


The methods above are usually the best way to control nitrate. Just muscle power and a good water conditioner to remove chlorine/chloramine.

That was nabi
I didn't even post because I thought it was such a good reply! Hope you'll get it all sorted out though... Just keep up with the water changes and hope for the best

If you wish to avoid spikes, which are pretty much inevitable in a small(?) quarantine tank, then add a product such as easyBalance.