I'm not really sure how the great stuff will handle being submerged long term. If you seal it with silicone that can only help though. The real problem might be keeping it submerged, the stuff is going to want to float. However, if you add enough substrate/rock/driftwood atop it, you should be able to hold it down.
Are you planning on using great stuff for the background too? If so, I would recommend spreading a layer of silicone over it first. A rougher surface will help it stick.
And don't make the mistake I did... Leave the false bottom in when you create the background, or at least, be sure you will be able to fit it back into place after it is done. I didn't think of this and it was a PITA to torque the false in there after the great stuff cured. Luckily, diffuser is a little more flexible than one might think.
This is looking good though. I cant wait to see the finished product. Any thoughts on inhabitants?
i'm hoping the foam will hold up under water but i'm also going to paint the foam black before i silicone it so that if by any chance the coco coir does come off you won't be able to see the ugly foam. i would rather just use black silicone, but unfortunately i can't find any that would be safe in the aquarium...
i'm thinking i'll just weight it down with rocks or something to make sure it doesn't float, and it'll be all siliconed into place too, so it shouldn't move around too much.
i'm going to make the background out of foam too and cover the back glass with silicone first, but i'm not sure how well it'll work. the tank doesn't have doors like an exo-terra so accessing it is going to be a real pain... and i don't think the foam gun will even fit in... lol i really don't want to use the straws... they just don't work.
lol that's funny about the background! i think i'll get the false bottom in place, then do the background and then put the pools in last. i think that'll be the easiest way to do it, so most of it is built outside the tank and then just placed inside it.
as far as inhabitants... i'm not sure... i think for the first little while i won't keep anything and focus on getting my plants to grow. at some point i will get shrimp (probably amanos) and i might decide to add frogs (either darts or tree frogs) later on, but i'm not loving the bugs requirement and the fact that i can't leave them to go on holidays without getting someone to come in and feed them.
i just spent a long time carving the foam and my back hurts a lot, but i think it'll look good when i finally finish
i also picked up a little pump ( hydor pico 400) and when i find a tube to fit it, i'll figure out the flow for the pools and then foam them all in.