60% to much? ive been tolded around there was about right, but whenever i get fish again i can change that. i drain the water then i add Kordon's ammonia cholamines and chlorine remover, then fill it again.
35-50% water changes per week is the best way......While setting the driftwood, you probably hit a bunch of mess under the gravel, causing an ammonia boom...........Try cleaning half the gravel every week by agitating it.........
do you vaccume the sand when you do a water change? I vaccume the sand every time and change 2/3 of the water in all my tanks every week and never had disease. I use prime as my water conditioner and add half before I start and the other half when the water is going in. I always use double the amount. (the prime bottle says you can do this safely without harming the fish.) I agree there has to be other contributing factors.
yaa whenever i get fish i try to clean the sand more often its probally been a month since i last have. Or I might just sell my tank eventually get a bigger one and tile the bottom so it looks good and no issues with cleaning.
hydrogen sulfide build up under the gravel is a possible cause then. When you stired the sand it was released into the water and slowly killed the fish. Gravel/sand has to be cleaned regularly to prevent the buildup!