I am new to fish tanks and I have a few questions. I want to keep angel fish I love them. I have two now I had four I also have a tinfoil barb, two silver dollars, pelco, siamese algae, clown loch, and two giant danios. My tank size is 55 I have a fluval 406 and sand as my sud, I have drift wood and a few rocks. I check my water every week everything is right on I also do a water change once a week of a few gallons. I have all types of food from flakes and pellets. Why am I losing fish please help.