Thanks for the reply. The tank in question was recently bought on E-something second-hand. It's only now that I realise that one of the corners has a chip taken out of it One part (about 5mm long) by the corner seems to be a meeting of silicone rather than glass. I'll rethink my plans, so I'm sorry for potentially wasting your time, and I'm sure that you'll find great homes for the toadlets!
All the best and have a great new year