like Wingin It said, i can almost say for sure that it was the vacuuming of the gravel that released ammonia and what not into the water which starved the fish of air. I personally do water changes 2 times a month. and have a bubble stone in the tank to keep the water airated.

Had the same happen and after tested the water and the ammonia spiked to over 30ppm. The guy I spoke to is a marine biologist and he said that when I stirred up the gravel that much I took out all the bacteria that controlled the ammonia causing it to spike to toxic levels killing most of the inhabitants. Now I never stir the gravel during water changes.