Sorry it's been so long since I posted - life just gets in the way sometimes!!

I'm about to re-do my 70gal wave-front tank & I want to layer up the back.
I know I've seen polystyrene used for this but I cant find any of the threads so I'm hoping you can help.
The idea is to build up different levels, not to make caves with it, & to cover it using pea gravel cemented on with black tank sealant - anyone see any problems so far?
Does it matter what type of poly it is? I've got some loft insulation type & it doesn't say that it's coated with anything... but it doesn't say it isn't -

Or do you have any other experience of what could be used instead?
Thanks for all your help - the Zebs are doing great by the way, not breeding yet but they seem happy & are growing well so I've no complaints!

Lucy <><