20 gallon tanks are a bit too small for african cichlids. The tank size is fine for grow outs but not for adults. Don't know what disease you were treating for as your link doesn't tell me anything. As far as losing fish right after a water change, it could be pH shock or possibly bad water. Do you use hot water from your water heater when doing water changes? Sometimes old hot water heaters can cause problems. I don't know the pH of your tank water or the water you use to do water changes. Also don't know the buffering capacity of your water.
I just wouldn't expect to see all the discoloration, scale damage, swollen anus, shredded fins etc. from water shock/ph swing. That being said the water lines have been redone in the last 6 months (but no on a time line that I think correlates to water change deaths, not sure though can't remember clearly) The hot water heater is also brand new was installed just prior to me getting the house.