I found a baby Cory in my HOB filter today. I have heard about this happening with canister filters, but never realized they could get into a HOB filter. Cute little critter. It kind of freaks me out to think that it was living in the dark filter eating scum. I suppose the upside is that this is probably how it survived long enough to not be eaten, but it still seems depressing. I will DEFINITELY be checking the inside of the filter with a flashlight from now on.
I did end up seeing the peppered cories mating, so I figured out that the eggs belonged to them. There was more than just the one batch as well. I wonder which batch this baby is from.