For whatever reason, I can't seem to keep snails alive in two of my three tanks. Not big, store-bought/raised snails, but the little hitchhikers that come in on plants. In my 25 gallon (convict, tiger barbs, upside-down catfish) and little 6 gallon (betta) tanks, the snails die within about a week, but in my 10-gallon tank (just a bunch of platies at the moment) they seem to be thriving. I can't measure any significant difference in water quality or parameters between these three tanks, and they're all kept at temperatures in the mid-to-high seventies. Why are the snails only surviving in the one tank?

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The little guys are probably Pond Snails. Many snails need hard, high ph water to survive. A local Big Box store couldn't keep any of their Mystery Snails alive (water was soft and acid) until they switched to hard, high ph water. Barbs also will eat snails.