Nitrite is rising no matter what I do!! Help!

My planted tanks nitrite levels keep rising no matter what I do. I currently have 2 blue rams, 2 platys, and 2 Kuhli loaches in my 20 gallon. The tank was fully cycled last week, but since them the nitrites keep rising slowly. I added CO2 yesterday, and this morning I tested the tank. Ammonia 0, nitrite 5.0, nitrate .5. I have been using seachem prime and ATM Colony since I noticed nitrite rising since last week. Since I’ve had plants in the tank I’ve put 3 doses of fertilizer into the tank since last Tuesday as I didn’t have CO2 and some plants were yellowing. I’ve been doing water changes constantly as well, but nothing seems to work. Any ideas as to what might be causing the nitrite?