Marble should be neutral to the chemistry o your water. It would look pretty neat as a back piece.If it is polished, silicone should stick without any problems. As for thickness, I can't say, and won't hazard a guess. Bob Dixon Cichlid Trader List Administrator http://cichlidtrader.listbot.com ------------------------------------------------------------------------- This is the apistogramma mailing list, apisto@listbox.com. For instructions on how to subscribe or unsubscribe or get help, email apisto-request@listbox.com. Search http://altavista.digital.com for "Apistogramma Mailing List Archives"!