red sponge??????

Couple things--anything that is leaching tannins? They can dye the AC sponges various shades. I have one that turned pink, while others don't change color at all.

Other option--cyano bacteria is usually a slimy red coating, and it could be coating the sponge.

I doubt it has anything to do with the neons...Sorry.
 
Is the sponge slimey, or just discolored? The discoloring is no big deal, but slimy could be bad news. I keep using my discolored ones with no problems.
 
I would replace the sponge if there is algae on it, especially if it's red cyano. Putting the sponge back in will mean all the spores get blown about the entire tank, where they will grow.

Sounds like the filter needs to be cleaned a bit more regularly.
 
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