What about using silicon from a DIY store. If you intend to have any wildlife in there, use silicon from a pet/aquarium shop.

exactly whta i was going to suggest andy. the pet shop stuff is the same as the bathroom stuff. check the prices for best value.

The bathroom stuff often has chemicals put in to stop mould and is poisonous so don't use it if you want to keep aquatic life.

the thing is that the normal silicon is only good for a watertight seal when used in conjunction with glass . on any other material it is just water resistant . there is a new silicone product that comes in black and some other colours i believe that will waterproof things . i forget the name but i got it from ridgeons and it was about £10 . my springer desided a while back that hed like to jump in my 2000 gallon pond after 10 years . when he got out he holed the liner with his back claws . i was beside myself thinking how im going to fix it . i found this stuff and glued a patch on at the water level while it was wet after cleaning the algi of with a kitchen scourer . touch wood its fine still . i believe some other diy shops that make a similer product like b and q . sorry i cant remember the name .