It's cleared up! After that last post I wrote, I went back down and glanced at the tank- like magic, the haze was gone. Still smelled bad though. So I went ahead and added more bio-media from the other tank's canister, removing that one suspect sponge (it really smelled sour, not nice and musty like the other sponges and media) and the dirtiest/clogged fine polishing pad. Feel better having a larger ratio of the filter box contents as bio-media for some reason. Also siphoned the gravel- gently. Did about a 20% wc in the process. Tank has never looked so better. Still an odor, but much reduced.
Fishes on the other hand... well most of them are exploring around. Fabio is laying on the bottom breathing hard. Smallest cory is sitting in a corner- resting? not sure. I am starting to be able to tell them apart, but feel reluctant to give them names. It nags at me that they still seem a bit thin- kind of tadpole-shaped when seen from above/below- wide head and narrow body/tail. I see them eat but only the largest (a female?) looks to be in really good shape. Maybe they need a parasite treatment.
I saw some interesting behavior from the largest cory in the last few days. If it's a she, doesn't appear plump enough to be gravid by my guess, but really seems it wants to mate. This fish darts round the tank pausing and flipping its fins, then up to another cory and quivers all over. Follows the other one close around the tank, quivering all the while and sometimes shoving up against the other fish or running right over it. This is also the one that cruises up and down the glass a lot- then pausing to quiver pectoral fins at the glass. I get the sense this cory is in spawning mood but as the others are unresponsive, is flirting with its reflection.
Also makes me think this is the only one of them who is really healthy. So now I have to figure that out.... I think I will soak their sinking foods. The snails swarm all over it before it appears to get soft enough for the cories to be interested. I'd rather feed the fishes first, and have the snails get the leftovers.