I just remembered why I never had fish as pets.

[Saddragon]

all jesting aside, it's the tank cleaning that gets them dead, simply ignore the tank and let the filters do their job. Only clean the tank once every six months. Add water as it gets lower. When you disturb all the dirt and junk you cause an ammonia spike that kills the fish.

When you clean tank do a ¹⁄₃ water change and don't disturb the gravel simply vacum all the "shit off the top with a special water pump."