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sealing my waterfall

Subject: sealing my waterfall
by Otter on 29/10/2008 14:48:37

My waterfall is a flagstone slab that is balanced so that it tips slightly forward toward the pond and the other end comes up to rest gently under the lip of my biofilter (what Aquascape calls a "snout"). A fair amount of water also runs off the back end of the flagstone, and I'd like to seal this gap, but I think it's gotten too cold for the usual black foam to cure. Please tell me what you think of these ideas or suggest something better.

Use a hairdryer to cure the foam.

Silicone. But I'd rather not shut off the pump down for 24 hours to let silicone cure. I started over just a month ago, and I'm afraid that if the filter dries out I'll loose too much bacteria. I do have a check valve, but it doesn't work well enough to keep the filter full.

Bentonite clay. Will it stand up to running water?

Foam weather stripping wrapped in polyethylene as a temporary fix. But will it kill fish or frogs?

Any thoughts?