I can only think that they would be leeches... any particular reason why you don't think so?
I've had leeches in my pond, they piggyback their way in on snakes and frogs and other visitors.

sounds like leeches to me they are very tiny when hatched and attach themselves to anything handy. Until they get a little size to them, then they live in the pots and bottom of the pond.

I would suspect they may be midge larvae. Check out these pictures

Here is a link that might be useful: google image search

I think I might have the same thing. It's not a midge larvae. I found mine when I brought some plants in for the winter. They weren't leeches either. They reminded me of the "potato bugs" you see in gardens.