I feel that tank is already overloaded especially considering the filtration you have available so I don't think a snail eating fish is the way to go. I would also avoid chemical means of killing the snails as the decaying snails will severely deteriorate your water quality. However there are two relatively simple ways to cut down the snail population to manageable levels. 1. Remove snails that you can see. Just pick em out and dispose of them, simple as that. 2. Cut down on their available food. They have to eat. Take away the source of their food (overfeeding is generally a big one) and you'll cut down the snail population by starving them. A combination of those two things will curb the snail population if not outright eliminate them. Unfortunately its hard to completely clear a tank of snails once it has been infested but in your particular case I strongly disagree with you adding another fish as it will cause more harm than good.