I think they might be magical
. Seriously Marisa...i wish i had a dollar for every time someone asked that question lol. We have covered this here a couple of times...but i dont mind helping you out with this. First off...your multiple snail question is quite simple. The majority of snails that are aquatic are a-sexual. Therefore you only need one..and presto..you have thousands a week or two after. Some ppl look at them as a pure nuisence...while others like myself have learned to live and create a happy balance of them. As to where they came from, its odd i know..but also facinating.
First off..these guys are super resiliant. Alot of them can survive out of water for a long period of time..while others die within hours. You stated that you did not add anything new to the tank which would have caused them to enter it..but infact you have...you have fed your frogs. Snails eggs are even more tough then mom and dad for the most part...able to survive often mixed into the very food that you are feeding your first. At the same time...it is possible that your frogs did infact carry these eggs...if your new snail looks small...well the eggs are even smaller. Here is my theory...laugh if you will..but ill throw this out to you and see what you think. I would believe that if infact it had not entered via a food source..these little buggers were possibly carried into the tank by either the frogs..a decoration etc...perhaps only one survived...the egg floated in the water..was sucked into the filter medium, and there it managed to turn into a snail..trapped but easily able to survive on the bacteria..etc inside the filter housing. There it multiplied...and either a tiny new snail made it out of the filter and into the tank...or an egg did.
Or!!! Perhaps they have actually been there for awhile and are a sand snail. These guys are often extremely tiny...and live almost exclusively in the gravel when the lights are on. When the lights go off..they come out and play
. Perhaps..there is infact hundreds already..but they are too small to see...the parents, or adults usually end up growing to a larger size and will often forge out during the day light hours. If your room mate keeps a schedule for light...ie..turns them on when she wakes up at "X" time...and turns them off at "X" time...then the snails have been on the same schedule and have managed to elude you
As for ridding them...or hopefully maintaining them...well thats another chapter that i will tackle for you if need be. In the meantime..you will get to watch the population of your new buddies explode into the hundreds over the next couple of weeks. I hope this has helped