snails
I used to pick those little buggers out of my tank by the dozens almost every day. They hitchiked on a plant I bought. Some things I have done-Look for the eggs and destroy them. They like to lay their eggs under big rocks, and on the inside of any decorations you have. Kinda looks like snot. Also, adding some freshwater aquarium salt (in the recommended doses) helped a little and the fishes like it alot. But the most success came when I bought some ghost shrimp. I've had the ghost shrimp for about six months and haven't had to do any snail harvesting in a long time. I think the shrimp like to eat the snail eggs. And lastly, when I buy a new plant, I take it home and do a water change into a bucket. I put the plant in the bucket for a few days, and then check it for eggs and snails before it goes into a tank. So, after about 4 years with snails, I've gotten the numbers down to about 5-10 in each tank instead of 50-100. On the plus side, they do eat lots of algae so I'm happy with the ones that are left. Hope this helps.