Sump

[kizno1]

I want to make a sump for my 15G bn fry tank because thye keep getting sucked into internals and im struggeling t keep the tank clean evan with daily waterchanges. What size should i make it and how many lph should the pump do. Whats the easiest way to match the amount of water going into the sump with the amount going back into the tank.

 

[OldMan47]

The flow match is automatic Kizno. The more you pump into the main tank, the faster water runs back out of the overflow to the sump. As long as you do not pump water back up so fast the overflow can't keep up, that aspect will take care of itself. A typical sump design number is to use a sump about 1/3 the size of the main tank.
I am struggling to figure out how a sump will help you though. Why not get a nice small external and use a sponge over the inlet to protect the fry from being sucked up? I use this setup on a Heterandria formosa tank to keep their microscopic fry from being sucked up. It is the replacement sponge for a major manufacturer's filter that I cut a slit in the end of and then slipped it over my filter inlet.
Here is a picture of the sponge

This is it in place on my Heterandria tank. There is an adult male swimming in front of it in the picture. He is all of about 2 cm long.

This is a shot from a bit further back so you can see how the suction tube is simply pushed into the slit in the filter.