Cory and snail problems

29-gallon tank
ammonia = 0ppm
nitrite = 0ppm
nitrate = 5-10ppm
water changes: 20% weekly with vacuuming
2 angelfish, 4 platies, (used to be) 3 cories, 6 live plants
temp 80 degrees

I have two issues with my current 29-gallon tank. The first and more serious issue is with my cories. I cannot get them to live more than 3 or 4 months - I just lost my fourth batch (I always have between 2 and 3 cories and lose them all at the same time). They appear healthy; no visible signs of illness or infection and don't appear emaciated, but after a few months all of them get very sick, sluggish, and unable to stay upright (they act similar to fish with whirling disease). They die within 24 hours of the onset of symptoms. No other fish in the tank get sick.

Has anyone ever had this phenomenon? Is it possible that they're not getting enough to eat? I put down an algae wafer once a week or so, and I always see them eating when I feed the rest of the tank. I don't vacuum under the driftwood; could there be gases down there that are killing them?

Other possibilities could be that my nitrate kit is bad - I've never gotten a reading over 10ppm. But the plants are thriving and growing and the angelfish spawn every 12 days or so, which are all signs of a healthy, balanced tank. How would I be able to tell if I need a new nitrate kit?

My other problem is with snails. I have both pond snails and Malaysian Trumpet snails. I've tried putting blanched lettuce in the tank at night, but the snails just snack on it and leave before I can collect any of them in the morning. I use my fish net to scoop out as many as possible, and I try not to over-feed so that they don't blossom. I still have way too many snails, though; they're great for cleaning up algae but they only add to the bio-load, and as mentioned above, I'm already concerned that my nitrates may be higher than they're registering. Would a dwarf puffer be a good idea? Are they even compatible with the fish I currently have? What do I do with it once I've gotten rid of my snails?

Thanks so much!