I like putting the sponge over the intake even without fry. It's like adding a little sponge filter to the HOB. Sometimes even adult nano fish can get caught in the intake. And sometimes I see my fish pecking at the sponge cover of the intake for little bits of uneaten food that get caught in it. Which, to my mind, is better than it getting sucked up all the way into the filter.I use sponge filters in my fry tanks, but with the HOB filters you can put a fluvial edge foam filter over the intake of the smaller HOB filters making it safe for fry as well. I have typically used Aqua Clear filters because they are pretty easy to setup with custom media and they are easy to find on the west coast of North America.