Update:

Using the Saran wrap had good and bad results. On the good note, it allows the foam background to just pop out once I'm done. On the bad side, it did have the odd effect (probably combined with the cold, as my tank was outside and the temp. dropped a bit that night, but didn't have good ventilation inside, alas) of pulling away from the back wall, basically going concave on itself. Normally wouldn't be a problem, but it puts a space between the half-pots I have set in their for plants, and allows more space for moisture (not wanted). So rather than stick the Great Stuff pipe in their and fill the gaps, I'm going to go with Black Jungle's more permanent method and just silicone the glass, foam it, silicone & coco coir the background.

FYI for anyone not sure if they want to try this or not:
-For my 50 G. tall tank it takes about 2 normal sized cans to make a complete background, eg. putting on extra so I have room to carve it back.
-Don't forget to silicone past where you expect the foam to be (and it does expand a LOT), razor it off later for a smooth background to glass transition
-Black Jungle said it was best to start in small layers. Given the size of their project, it's probably safe to assume they meant go through a can, let cure, another can, cure, etc. I highly advise NOT stopping mid-can to do this. Even a little drop of foam in the 'straw' of the dispenser will clog it up and be a pain to get out.

Another word of warning; if the dispenser is clogged, don't try to force it out by pressing down more. This only results in a good deal of pressure behind it and when you finally DO take off the dispenser to clean it (wire works best), well let's just say I had a good deal of fine foam covering my front. Stayed pretty clean up until that point :(

Good luck anyone else, I wish I could post pictures of this start-to-finish as I repeat this process, but alas no working digital for me.

Hi
Have used the "stuff ' on several projects over the years and the only situation where it let go was in 25 inches of water.lol Took almost 3 years to do so though and then only on the bottom. I've found that if it doesn't adhere to the surface it tends to curl. For me the best method has been to lay out a design on a table top form it ,let it cure then carve the back side flat.Then paint it and the silicone it in place.
i used it to make a "seep" wall in the shadehouse. It is now 10 feet wide and goes up the wall 8 feet so far. Was really tough to paint.So am now using the table method for additions. Have found all the orchids will actually grab hold of it and the succulents can be just stuck in holes with a bit of sphagnum moss . Made planting pockets in the wall by cutting flower pots in half and foaming right over them So far have not found the ideal potting mix for terrestrials but find i need no media at all for epiphytes.
Have also discovered the cheaper variety of foam shrinks with age while the "great stuff" does not ,even underwater. gary

Hey Garyfla! Hows things!? I like eggcrate, just cut it out to fit the insides of the tank, remove it and spray the foam on it, let dry, slide it back into the tank, cut the foam at the corners to get a good fit, when you got that done take it out and cover it with silicone and coco fiber =) when thats done cover it in your choice of plants and slide it back in the tank =) BAM! removable foam background! ; )

Like the ideas tried for removable backdrops! Main thing that held me back for doing a backdrop with GreatStuff or anything similar is that I did not want it to be permanently attached.

Thanks for posting your ... not quite successful attempt, Jordan. It's good to know what DIDN"T workout well as much as what did!

Gary and ILT, thanks for letting us nkow what you found worked! Both of you mentioned "carving" the foam. How difficult was it to do and HOW did you do it?

ILT... I REALLY liked the eggcrate idea! Seems like that would also provide a bit more structural support!

Its very, very easy to cut, just use a sharp knife and cut the corners in an angle to fit, kinda like cutting floor moulding, in an angle so the two corners can fit ; )

Hello
ILT Great idea about the eggcrate . Would give a lot of strength and you could make it any size. You could also paint ,plant or whatebver outside the tank!! I'm going to use that as a base for some hanging planters.Thanks I've used sheet styrofoam as a base, works but no strength at all.
As to carving I use a serrated plastic knife ,works on both styro and expanding. foam without all the beads. Potato peeler works also for sculpting.
Some other things I've learned about X foam . The cheaper stuff shrinks and tends to let go . particularly underwater. Does not hold paint nearly as well as the "Great stuff" Also the triple expanding type goes further.
I used it several years ago as a background for an African Chichlid tank using real rocks as a base . Styrofoan was too bouyant, no silicone required. The arylic paint has held just as well as the two part epoxy at a third the price.
One new revelation on the large tetra tank. Chinese algae eaters love acrylic paint . lol Has proven to be non toxic but they have completely cleaned the pipes lol.
Also discovered that epiphytes will root directly into it
I intend to make an epiphyte tree and use this rather than cork. ,with a base of PVC pipe. Have used real trees in the past both living and dead with many problems . This should solve them all.
Only warning on the foam DONOT get it anything especially yourself . It will not come off!!! gary