Snails. Where The Heck Did They Come From?

The usual cause is live plants but as there aren't any, that's not the reason here. However, you say you have another tank. Do you have live plants in there, and if so, do you use the same equipment on both tanks? It would be quite easy to transfer snail eggs that way.

If you don't have live plants in your bigger tank, I can only think that they arrived with the tank and have been in hiding since you got it. Assuming you have those tiny ramshorn snails, all you would need is two snails tucked away somewhere and you have enough to start a new population. Since they are hermaphrodites any two snails would do it, you wouldn't need one male and one female. One website reckons that even if you completely strip a tank down you only find 10% of the snails in it.

Cucumber might be a better 'trap' for the snails - and don't forget that whichever veg you use, you have to get up while the room is still in total darkness to find the snails on the veg. One thing you could try is using a screwtop jar for the snail trap. Punch snail sized holes in the lid from outside to inside so the spikey bits stop the snails getting back out. Put your chosen veg inside and leave in the tank overnight. In theory, the jar should be full of snails in the morning. Be careful that the holes aren't big enough for a fish to get through or they'd damage themselves on the spikey bits.

Or you could just leave them there. That's what I do now after several years of trying to eradicate them. They aren't actually bad for the tank, just unsightly.

They are hardy little things! I've recently restarted a small tank which had been sat empty for about 2 months (but with gravel still in situ) - it had a minor small infestation prior to being shutdown and when I refilled it quite a few snails started mooching about again as if nothing had ever happened!