Thanks for the replies!
The aquarium silicone I bought is actually black! I've decided I'm going to use the regular black silicone to coat the glass (where the foam will go) and then use the black aquarium silicone to stick the coco fibre to the background as it will guarantee no harm will come to my crested gecko. Also I'm going to coat the false bottom layer with the aquarium stuff as it will have contact with the water system.
Heres to hoping 2 tubes of regular stuff and one aquarium tube will do!
As for the foam, I've found a few brands but wasnt sure which would be best. I'll work it out
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Anyone in the UK know where I can pick up egg crate? Do many aquarium centres sell it? B&Q didn't have any!