Hey, it's a 90 gallon tank. All you do it take all the measurements in inches and multiply thme together, then divide by 231. example: 36"x24"x24" = 20736/231 = 89.766233 gallons.
What does your frame look like? Are you able to slide glass down into it or is it just pieces of wood/metal connected to make kind of a box? You could silicone glass/acrylic/plexiglass inside the frame thne make sure to run silicone on theedges inside to prevent leaks. David Taylor has a good site with a stream and a nice hex tank. I'll put the link at the bottom. There's many ways to make streams. i'm messing around with my digital camera and I'll try to get a picture of my stream up. It's jsut gravel on the bottom, then the stream area is lower than the area with moss to create a low stream. Moss is now growing into the stream, looks good. Good luck.
David's Page
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P. vaillanti
derekb15.tripod.com/tropicaltreasures
4.2.0 P. sauvagei
3.2.7 P. hypochondrialis
2.0.0 P. vaillanti
0.0.3 P. aurotaenia
2.2.0 B. orientalis
0.0.10 S. pustulosa
0.0.3 B. americanus
0.0.1 M. laevigata
0.0.1 M. milotympanum
1.0.0 T. horsefieldi