Help, I thought I cycled my tank and the plants are thriving. I bought 2 endlers, and they seemed to do fine for a couple hours, I left for an hour after that and came back both fish were dead, I was only gone a few hours. So somewhere between 4 & 5 hours they died. I didn't do an ammonia test but I never had fish in there, so I stuck a Tetra from another over populated tank in there and this time the fish died within 10 minutes. I did another strip test, nitrite was 0, there were some nitrates. I only have an in tank foam air filter.... What am I doing wrong?