Over Feeding.
dont feed too much, dont feed on daily basis, keep your fishes hungry is better than feeding em too much and watching em dying day by day.
feed a day and dont feed on the next day, only once a day. keep the tank clean and away of dust or heat. when u change the water be sute its cool water or atleast same as the room’s temp.

the trick is never get the fish outta the tank
our’s used to die really quick, so taking the fish outta the tank hurts the gills, and so you just scrub the tank while everything is in tact and then clean the filter.
After an hour the filter will clean all the gunk that was stuck on the glass.
we’ve been raising fish for about 10 years