I'm not sure the kreisel size is so important. Any larval tank needs to be small enough to concentrate the food but large enough to be stable. So while most people use a stand alone tank and start with a small water level, I prefer to put the larval tank in the broodstock tank. Float it shallow if you need to concentrate food for a few
larvae, float it deep if you need to concentrate food for a lot of
larvae. So the key is the relative # of food organisms per
larvae per ml, not the total # of food organisms.
BBS can be sorted by screen size, but because
BBS grow so fast relative to your
larvae, the
BBS need to be removed from the larval tank, or they compete with the
larvae for food. A good idea is to keep lots of greenwater phyto in the tank in any case to help the
larvae see their prey as well as to keep their prey nutritious.
I think the best kreisels are shallow round black plastic buckets with screened bottoms. The mesh size has to be small enough to keep in
larvae and should be changed out for larger sizes as the
larvae grow, because you want any smaller waste to pass through the screen. But most people don't agree with the approach of combining broodstock and
larvae water, so there are many other ways also.