+1 some sort of infection.... Maybe leave your tank fishless for a week or two and try again. If things are still bad maybe think about treating the tank...
it's doesn't really matter too much what pump you use. It all depend on how fast you want the water to fill up your tank. This is a good idea and you should do it. If I were you I would just get a 7-800 gph pump and not even put a valve on it. When the water is getting close to filling up just...