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I have 2 plecos in my 150 gal. They're the most relaxed fish I have ever seen, until I put snails in the tank. They have already killed 3 or 4 of them. The second the snails touch the gravel, these plecos go into kill mode. Is this normal?

Depending on the species of Pleco, yes, it's normal.  Most of them enjoy meaty food once they're adults, and snails fill the bill quite well.  If you put in large enough snails they should be OK, also be sure your plecos are getting enough food.  Sometimes the other fish in the tank intercept too much of the food for the plecos to get enough.  If that's happening they might literally be desparate for food.