Hey. I am really sorry about the troubles you've had. No matter where the fault lies, its hard to swallow when one is an animal lover, especially if one tends to blame themselves. Everyone can fault their fish one way or another but not everyone is grown up enough to admit it.
My sincere advise would be to add more filtration as smjack mentioned previously, and try to develop a habit of changing at least 50% of the water weekly at once. You won't go wrong with that, and even if you do nothing else but drain and fill, weekly feed the fish daily or every other day, they will be fine and healthy. Water changes is the key, not even filter maintenance is of such importance.