Belly-Sliding Fish Fry: What Does It Mean?
Fish breeders will notice that healthy fry swim about in the tank from the moment they become free swimming. Other fish fry sink to the bottom and slide about on their bellies, picking up food from the bottom as it falls there. This may seem like fairly sensible behavior for fry, but it is usually the first sign of swim bladder disorder, and in most fry it means that culling is the best solution.
Swim bladder disorder occurs in baby fish fry when the organ that controls the swim bladder fails to develop properly, leaving the fish unable to swim normally. Some fish do develop swim bladder problems later in life, often due to overfeeding or illness, however the type of swim bladder disorder that is seen in fish fry is not a sign of sickness so much as it is a sign of a lifelong condition the fish will always suffer with. For this reason, belly sliding fry should be eliminated from the breeding population and preferably, from your fry tanks altogether.
If large numbers of your fish become belly sliders, you may want to look at the types of food they are getting and the condition of the tank water. BBS, or baby brine shrimp is often associated with swim bladder disorders in fry, though some breeders claim that feeding baby brine shrimp has no negative effects at all. I can say that in my own experience breeding betta fish, spawns fed extensively on baby brine shrimp certainly do have a higher rate of belly sliders than spawns fed extensively on other foods like microworms, in which instances fish seem to develop other issues like missing ventral fins.
It is incredibly important that your baby fish fry have clean water, and that baby brine shrimp are cleaned of their egg shells before being fed to your fry. You may even find that you get better results with egg yolk rather than BBS or micro worms, although breeders who only use egg yolk often find that their fish grow more slowly and have a lower survival rate than those fed on live foods.
If you have bellysliders, give them a few weeks of good clean water and good food. If there is no improvement and your fish is a confirmed belly sliding baby there is very little you can do. Some recover partially to have some swimming ability, but unless you have room in your tanks for several fish without proper locomotive skills, it is kinder to cull them. As they grow, swimming may become increasingly more difficult. Culling is the best and kindest option you have.