In my most recent tank, I used hydroponic Coco-Tec slabs, which are about 3 inches thick X 6 X 36, but can be easily cut, divided horizontally and put together to be deeper than 3 inches, if you want. They are made from a loosely woven cocoanut fiber which has been stiffened and further preserved with what I believe is latex. You can cut it and shape it to fit the tank, silicon the pieces together, cut out ponds, a place for the pump, etc., then hid the edges that go against the glass with aquarium pebbles or other rock. Water flows freely through it, and it supports the substrate without sagging. (This particular tank has been set up for a year with no problems so far. It is totally neutral and non-toxic.) I put a layer of weed cloth between the Grow Slabs and the substrate, because it is rather coarse and the substrate will filter into it if you don't use a finer barrier to separate the soil from the water.
Depending on where you live, you may be able to find this at a local nursery or in a large city, there may be a hydropnics specialist. My last shippment came from CropKing, 5050 Greenwich Road, Seville, OH 44273.
For backgrounds, I always like to incorporate cork bark. Although I haven't tried the Great Stuff yet, I've seen it at the AZ frog show, and it looks like it works well. It would also work great to hold cork bark, so I'm thinking of using it in my next vivarium. Whichever methods you chose, have fun.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
7 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
6 P. terribilis mint and organe
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus
2 P. lugubris