Okay, I made a makeshift sort of breeding trap with an old plastic container and put my betta in, and after I fed him he was pretty okay with being in there for around 20 mins or so. But the corys didn't touch the food at all, even though one of them swam near it and sat next to it for a while. I'd been quarantining them before this, and when I fed them in the quarantine tank they wouldn't touch the food within the first 30 minutes, but after a couple hours the food was gone, and it looks like they might still take that long to eat. I'll try with another kind of sinking wafer tomorrow, but I'm worried that the problem is that they just take a really long time. I suppose I'll see how it goes though, perhaps in a few days they'll get hungry enough to eat quickly.