Snails very unhappy

[ScubaSteve]

Nitrates: 0ppm (just tested)
Temp range: 78 to 80F, but is pretty much constant at 79F

My cerith, top and trochus are the ones having the greatest issues. Margarita's are kind as they always are: stupid and getting stuck upsided down.

All of this started after I moved and went into a larger (40G) tank. When I had all of this in a 20G with a high bio-load and no skimmer, it was EXTREMELY stable and had zero issues. I go to a newer, bigger tank (which in theory is supposed to be more stable) and BAM! all this crap starts happening. It's weird man...