First I have to say, stop buying anything else until you educate yourself more. You just killed a creature that lives passed 100 years in most cases. They should never be added to a new tank. If it was only 3 inches, other stuff is probably fine, change water to be sure.
In the future, don't buy things on a whim. Read for at least a few weeks, everyday, before buying something. If you can't recreate their environment, don't buy it. If you "have to have it" setup another tank and realize you're going to have that much more work to do.
Sorry for your loss, but not really, it sounds like you knew nothing about the creature. The first thing you needed to do was get some covers made for your powerheads. Yes, covers are ugly, but they keep the critters safe and house filter feeders as time goes by.