new additions to the tank- accidental

So I went on vacation for a week and left the cheap 7-day feeder blocks (that have krill and spirulina)in my 10-gallon 1-fish tank. Here it is 3 weeks later and I noticed a few tiny snails (what I am assuming are snails)on the sides of the tank. Where did they come from?

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Most healthy aquarium fish can go without food for at least a week, so the feeder block was not necessary. It also didn't introduce the snails unless you took it from another aquarium. If you have live plants, the snails and/or their eggs probably hitched a ride on them. That said, live plants are a great help in keeping an aquarium balanced, the snails in reasonable numbers may help and don't hurt anything, and their population is easily controlled by a lettuce lure. Float the white part of the leaf for a couple of days, and remove it when it's full of snails. Swiss chard and some other things also work as a snail lure.

I have nothing live in my tank except for my fish. And now some snails. The feeder block came from the package I bought it in. I have marbles for gravel, a number of fake plants, a heater, and one Paradise Gourami. He has been in the tank for about 5 months and I have never noticed the snails before. Any other ideas where they could have come from?
PS- before I went on vacation my tank was getting kind of green and I planned on giving it a cleaning when I got back. Well when I returned it was miraculously clean. This is why I assumed it had something to do with the feeding block.

The snails or their eggs have to have ridden in on something wet. Some species hide in the gravel and are nocturnal. You see them only when they over populate the aquarium. All that algae and free food must have contributed mightily to their baby boom. Live plants will help control algae, and regular, partial water changes will remove the nutrients that snails and algae need to thrive. The problems should go away if you do a weekly, partial water change of about 20 percent and bait the snails to their ultimate dehydration.