Well the tank smells pretty bad too (same smell as the one on the sponges). Swapped the bad sponge for another ok sponge and still got the old media in there. Added a carbon sponge too like you said so hopefully this is gonna sort it. I think its been that sponge.
They should perk up now, what have you done with the sponges as I would take them to the store, keep them in a plastic bag.
You might be able to claim for the fish if any die, as is sounds like the sponges havent been stored right or something.
I can't really remember where I bought them to be honest. I think it was acorn pets. I probably don't even have the receipt. I still have the sponges but theres nothing I can do. I think the snails are dyeing now too.
I have a cupboard under the fish tank, where all the fish things are kept. Sponges were in their box but air can get in and out if you know what I mean. Cories are still going up to surface, and still slightly lopsided. But still not breathing properly. This is horrible. If my fish die I'm never keeping a tank again. This is too heartbreaking.
I would turn the filter up to full speed, sorry hang in there.
As with the snails and corys being the worst effected sounds like you need more aeration at the moment.