20 Gallon Angelfish Tank

This is a 20 gallon tank that has been set up since August 2003. It has housed angelfish since its start-up. At first, I couldn't keep an angelfish alive in here, now I can't stop them from laying eggs! After the first few tries, they got it right and I found myself taking care of a tank of fry. The father actually took the dominant care role with the fry - moving them to another cleaner location when their tales had emerged, occasionally cleaning them, and keeping them together in a group at ALL times! Once they were free-swimming, he was racing around the tank, swallowing them and spitting them near the group. I had removed the female from the tanks as I was scared that dhe would get mad and eat them as a snack. I had 12 babies that made it to the size of a pea, and only lost one shortly after that. Out of the 11 babies, I had 6 that were predominantly white, and 6 that were predominantly black - they came from a black marble male and a purw white mother. I kept 3 babies and gave the rest away to a reputable, local pet store in exchange for some food. I noticed that with the ordeal of laying eggs came parasites - they are tiny, hairlike white things that grow when there is enough stuff in the tank for them to develop with. I noticed them first when the female laid a batch of eggs and the male was not successful at fertilizing them. I cleaned out the unfertilized eggs in a timely fashion, but that didn't seem to stop them. When the fry were about 3 months old, I transfered everyone out of the tank and cleaned it thoroughly with aquarium salt and hot water. I transfered the mother, onr of her babies, and a striped angel in the tank. I still find that if I do not clean the tank (filter, glass, syphoning the rocks), that I will notice some parasites starting to develop. (I have not noticed the parasites in my other angel tank at all). Witnessing the fry develop (and making sure they were fed) was an awesome experience, but I keep the two mates apart as the care involved in raising fry is very high, and I do not have any room for more angelfish! I have a Fluval 2 Plus filter in this tank, as well as an air stone. It also gets cleaned weekly, if not every 5 days.