I have a 20 gallon long tank and last week I got 6 cories, 6 rasboras, and my betta to go in it. I tested the ammonia everyday and it is at 0, but today I tested the nitrates and they were close to 90! I'm going to do a major water change today to bring them down.
Before I added the fish the nitrates were always less than 5 and I never had problems with them. Now in just a week they've gotten super high. Does this mean that my tank is overstocked? I asked in several places if this stocking amount was alright and everyone said that it would be fine but I don't think it is.
If I do a water change do you think the nitrates will just shoot back up in a few days? If they will, I think I will have to return some fish. What should I do about the nitrates and which fish, if any, do you think I need to return?