they make exterior UV resistant pond spray foam.
This is what Im going to use. I decided I am blacking out the back of the tank with tint material again (limo dark tint, just ordered) and then Im going to make a more sectioned BG.
Yours looks awesome though, DEF something Id actually pay for!
If your decide to make a thread let me know, I'd be interested in following . Actually I was thinking about making a little summer business out of it. Maybe find some people that want one and give them different design options , I'm not sure yet there's to many variables I haven't thought out yet
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