I have a waterfall, which flows into a small stream constructed with
rubber liner, that then flows into a small pond.

The pond liner is (I think) some kind of cheaper plastic looking liner.

I believe water is leaking where the stream liner overlaps the pond
liner, ie water is creeping back up between the two liners.

I was wondering what is the best way to make a seal. I know you can
cement rubber to rubber, but I'm not sure about rubber to plastic, I was
considering using silicone (normal bathroom silicone) as it seems to be
very sticky on both types of surfacea and I already have a coiuple of
tubes, but I wonder if this could have harmful chemicals for the fish.

Any other things I could use?

(While writing this, I just realised that aquarium sealer must be safe,
but would it be flexible enough?)

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Best regards,
Ms Fish