Keep an eye on t he size and the number, they're fine as long as they're just eating your algae, but they can become an infestation and when they hit that they're nigh on impossible to get rid of and then you basically have to scrap everything and start again.
If you're adding new fish by acclimatising and then pouring the lot in, they probably came in as microscopic things from wherever you got the new fish from and took a while to find conditions amenable for growing. If you bought any new plants recently they might have come in on the plants too.
You know, if they could take a while to find conditions amenable for growing they could have came in on the last couple of plants. Iremember the Little Lady going on about an air bubble on one of the leaves for ages and me kinda doing that parent thing going Oh yeah, like air bubbles. Fancy that
I guess my fears of becoming some kind of hell-bound snail killer are finally going to come to pass! :OD
If they were tiny, transparent and not actually air bubbles they may have been snail eggs XD
That sounds like them. I am now cursing myself for not pouring bleach and alkaseltzer into the tank!
Oh my that's a bit excessive?! XD
For future reference depending on the plant, you can give them a gentle wipedown to get rid of any eggs before adding them to your tank :)
I am an excessive expressive man!! :0D
I shall remember that. I genuinely had no idea that things could hitch a lift but of course, it makes perfect sense