Malaysian Trumpet Snails dying?

Hi all,

So I recently got a 75gal tank from a friend that has a large pleco (that needs a new home), 2 large featherfin catfish and two eels (a tiretrack and a yellowtail). There were also hundreds of MTS in this tank. Until a couple days ago, they were everywhere! Now, I hardly see any of them moving, I see some dead ones on top of the gravel, and there are NONE on the glass, even after lights out.

Recent changes:

I work at a pet store. Someone brought us some larger cichlids to rehome so I took them and put them in my tank. I added too many at once, I wasn't thinking. Yesterday I had completed two 50% water changes before noon due to a nitrate spike, and added an air stone because they depleted the oxygen in the water. Dumb move, but I have the fish now, they are doing well and I will do water changes, etc. as needed until the tank stabilizes. I dechlorinated the water of course, and I also added a small bottle of safe start to the tank to give the bacteria a boost.

Having all these new fish, I also added a few pieces of driftwood to the tank to make hiding places, along with a large terra cotta pot. One piece is mopani wood and the other is just the standard $5/lb type of driftwood I got from my LFS. When I first put the wood in, it was covered by snails within an hour (just a few days ago).

Any ideas to why my snails are all disappearing/dying? I don't see my fish messing with them. Could it just be because the fish are eating all the food put in the tank and there's no leftovers for the snails? I had hoped the mts would climb all over the new driftwood and clean it all up a bit but I haven't seen one moving in two days, and I kinda liked having so many! I put some algae wafers in the tank and I see a few moving around, but that's it.

Tank stats:
Temp: 75*F
pH 7.5
Ammonia: ~0.25ppm
Nitrite: 0ppm
Nitrate: 40ppm
(values before 20% water change done after testing)

Thanks!