It can take a fish days to die it's awful, do you feed any other foods besides tetra mini flakes, as the need a varied diet like frozen bloodworms, daphnia,brimestrimp,river strimp, white strimp,I only use freezedried tubilex worms with them not being to safe, peas,cucumba,lettuce,broc,etc, as daphnia is good as it helps the fish digest it's food, and peas are very good in the diet.