I put some pond water in my tank because it had larva in it and I introduced snails. Small and black, and they are going to town laying eggs etc. How can I kill them and not harm my mosquito fish?

I suggest living with the snails. They shouldn't harm the mosquito fish. If you want to kill them with chemicals, there's a copper-based poison called "Had-a-snail." It shouldn't harm the fish but will harm most invertebrates. I list some other ways to control snails on my snail page at http://www.fishpondinfo.com/snail.htm

You can also put in a clown loach. They are nicely colored and LOVE to eat snails. Their only drawback is that they can reach 10 inches sometimes. It does take a few years to get there though.