I have an in-tank filter that I'd like to use for a breeding tank, but I'm worried about fry. Does anyone have a material they recommend to use sort of like a sponge filter, to make it safe for fry? It's a large-ish unit with the pump in the bottom. Maybe close to the size of a 1 liter bottle.
it would help if we knew the specific model/brand filter you are referring to
like locha said, there are sponge's out there used for other filters that would work if its an uptake tube. what I've done is bought a large sponge filter insert for some zoomed canister, it already had an area cut out in the middle of the foam for a tube, so it works perfectly over filter uptake tubes and even over power heads. the foam was even so long I cut 4 inches off it and use the two pieces seperately.
if you're considering being serious about breeding you may just want to buy an air pump and a sponge filter, much easier to deal with down the road.
If it's a tube-style intake it's best to use the biggest sponge prefilter that works with your tank and your aesthetic. The bigger size slows the movement of water into the intake, which keeps it from getting jammed with debris and allows even small slow-moving fry to avoid being sucked in. Make sure to use open-cell foam. Either get a designed prefilter or cut a big piece of appropriately small-celled foam.
"In the end we will conserve only what we love. We will love only what we understand. We will understand only what we are taught." Baba Dioum