Snails

Just discovered two baby snails in our tank. Is this good or bad?  And how did they get there? I'm guessing they came with some of the fish we put in.
Update, after reading other posts on here (should have done that first really) I have the answers I need.

Snails are actually a good part of the ecosystem in a tank - provided they don't over run the tank. If they do, it is a sign you are feeding the fish too much as they feed on uneaten fish food.

The most likely 'pest' snails are the ones usually called pond snails or trapdoor snails. They lay their eggs in blobs of jelly which are very hard to see. They usually arrive in the tank on live plants, if you have any of those.

New fish are usually quite stressed by the time they arrive in your tank - they've been through a lot in the previous weeks from the wholesaler to being bagged up for you - so it takes very little to 'push them over the edge'. If there is anything above zero for either or both ammonia and nitrite, that would be enough to do it. If you do have readings above zero, don't get any more fish until they are at and stay at zero.
And platies are becoming more and more inbred which tends to make a species less hardy.

Platies are not shoaling fish so the last one should be OK on its own. Is it male or female? If it is female you will most likely have fry - every month for about 6 months as livebearer females can store sperm.

If you do succumb to multiple tank syndrome and get a bigger tank, you could have several platies in that but make sure there are twice as many females as males.