Hello! While feeding the fish today I noticed one of the cardinal tetras is missing most of its fins! he was swimming erratically and didn't eat at all. A little info: Tank is a 20 gal that has been running for 2 months now, it was cycled with pure ammonia over a month. ammonia and nitrite are 0, nitrate is <20( tested w/ API liquid kit). I do 20% water changes bi weekly and try to clean the pool filter sand with the siphon. Filter is an aquaclear 30, and I have 2 15w halogen bulbs in a canopy. The tank is stocked with 2 GBRs ( a male and a female) and 10 cardinal tetras, which seemed compatible according to everything I've read so far, as well as aquaadvisor. Ive had the tetras for a month now and the GBRs for 3 weeks. The tank has a mix of real and fake plants, as well as some malaysian driftwood. Here is a video: https://www.youtube.com/watch?v=5oVi8EbBfN0 I have been having problems with this tank, the GBRs looked like they were a pair when I bought them. I bought them from a really good local LFS, he sat and watched the gbrs for some time before he picked them out for me. After a week of relative peace, the male(http://i.imgur.com/4mDnHrZ.jpg) started chasing the female(http://i.imgur.com/7FtyVUQ.jpg) all over the tank for about a week, then they switched and the female, who is quite smaller, started chasing the male relentlessly, and nipped at his pelvic fins(http://i.imgur.com/N8aaQ53.jpg),which has healed since then. Now they switched again, and the male is chasing the female, the aggressor always seems to be vibrant, while the other always gets stressed and lose color. Is this normal? I'm new to keeping cichlids, I understand they are territorial, so I took them both out of the tank and re arranged the whole thing, making sure to create very clearly marked territories, but it didn't work at all. They only seem to chase when the lights are on, so I have the timer set to 4 hours on, 4 hours off, 3 hours on to give the fish a break. Last week I found a dead tetra that was missing its fins and both eyeballs, and now the one in the video seems like it might have the same fate. Are the GBRs doing this, or is it a disease? or maybe the tetras are fighting with each other. The LFS I bought them from is willing to take one or both of the GBRs back if comes down to it. Any help is appreciated, thanks for reading my wall of text.