Some snails are very beneficial in your tank by way of removing algae, but as is mentioned can become overwhelming. Se if you can figure out what type they are, then do a little research, It could be you might decide to let them stay awhile
If you like having some snails but don't want them to take over I have had some success in the past by introduction of snail predators. Depending on what type of tank you have there are some options. I have used round gobies (an invasive species here that is easy to get but I am unsure about the legality of keeping them), crayfish (very cool critters but will try to eat your fish and will occasionally succeed). There are also a host of very attractive fish that can used to feed on snails. Such as
Many types of cichlids and discus (Do some research here there are hundreds of them).
There are tons more but thats all I can remember off the top of my head right now.
Oh and if your going to go this route you need to make sure the fish are a litle hungry. Too much prepared food will often make them lazy and they will not touch the snails.
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