@Non atheist: how high do you reckon i could go before pressure becomes an issue?
@instanthhc: By shelf i meant, thats what it was used for before i had this idea, kind of just used to store stuff on it, although nothing as heavy as a fish tank.
Also having had a look at it, the side profile shape is made of 2 bits of plywood so extending it would be possible, however, the flat surface is atm made of one width of plywood, would adding a second peice to edxtend width comprimise structural integretey?