****, i had a whole article typed on the subject and i guess it was too long. so ill start again. I have my 90g cichlid tank. 1-7" fh, 1-f1 labiatus at 4", 1 female festae 3" and 1 dovii 3". I feed two to three times daily with nls pellets and freeze dried krill. occasionally i throw in earthworms or crawdads for some fun live food. I have 4-96w pc flourescents, and its right next to a window recieving 2 to 3 hours of good sunlight most of the year. My water is crystal clear, and i do 50 to 90% water changes weekly. I am with peacock. the only way you can do too big of a water change is if your fish arent used to it, causing it to shock them and bring on stress from too drastic of a change. most of the time that would mean your tank is too dirty, but your fish adapted to the environment over time, thinking that you are doing a favor by thoroughly cleaning the tank, you remove all the old dirty waste and throw in nice clean fresh water and shock the fish because the change happened to quick and they didnt have the time to adjust. if on the other hand once your tank is cycled you begin performing large scale REGULAR water changes you wouldnt have the problem as the fish would adapt to what was happening in their environment. For your algae bloom, you definitely have way too much nitrates in your water. Not doing big enough REGULAR partial water changes.