My tank is 200 litres, Tetratec External EX1200 running, C02 injection, fairly well planted planted and I dose TPN+ at the weekly water changes. Stock is as follows 1 plec 6 Red eye tetra's 5 platies 6 Panda Cories 3 Congo tetra's 10 Cloud minnows 10 Galaxy rasboras 1 molly Various inverts Tank cycled for around two months then I added the top half of fish and then added the rest over regular intervals, weekends etc. Not all at once! I spotted the fish gasping at the weekend and did a API check and the levels were fairly high did a 50% water change and they were still high did another last night and they were still high but better than before around about 1 ( ppm ? ) for both ammonia and nitrite. Is it over stocked or is my filter struggling to cope with the ammonia my fish are producing? Im intending to do another big water change tonight to try and curb the levels getting to high.