Why do they do this?

[Miffy moo]

my babies are growing very well, average 1.5cm tip to tail start, and appear extremely healthy( boy can they eat!) one particular one however occasionally lies down, always under sponge filter, for around 5 minutes or so and then gets up a swims away normally. It's one of the larger ones and otherwise appears in good health. has been doing it for a week or so. I have given them a full cycle of waterlife sterazin for gill flukes as a precaution more than anything, reading that gill flukes are enemy number one. Any ideas? Thanks :?

[boxters]

if it is lying on its side and swims funny it could be what we call a belly slider. This is not an illness but a deformity. It would be best to cull it