I have a man-made pond with a bio filter and waterfall. Somehow, probably from adding plants to the pond, my pond has gotten some underwater pond snails in it.

I went to open my bio filter for its weekly inspection and I noticed that there are atleast 60-70 snails (probably more) that have bunched up on the coarse filter pad right at the water-line. The biggest snail is as big as my pinkey-finger fingerprint, shell and all. They are all alive.

Is this normal snail behaviour? Are they trying to get out of the cold pond water but dont want to try dry-land?

The pond water flows into the filter box and is pre-filtered by a piece of chicken wire I put in there to catch the super big leaves.... then there is a net in the filter box and then this big green coarse pad. I need to hose off the pad every once in a while to 'clean' it. But if I do this with all the snails on there - they'll end up in the yard and will die.

Any thoughts or advice?

The snails may have gotten sucked in and then are trying to find a way out. They don't want to leave the water. If it's slightly warmer in there, that could attract them so they may have crawled in on their own. In congregating near the surface, they may be trying to get more oxygen as the filter may be lower in oxygen in there perhaps.

When you clean the pad, just pluck the snails off and toss them into the main pond. If you don't want them, then discard them. I always feel bad if I do that so I just toss them back in. My 20 gallon aquarium filter becomes full of trumpet snails when I clean it. I can only save the larger ones; the babies are too small.