Cut the two pieces of plexiglass to fit the right height you want it, and in this case, somewhat longer than the front to back 10 inches, to allow for whatever curve you want in the pond on either side, and to leave a viewable portion in the front. Like parenthes. It can be bent by heating it in the oven, then bending and fitting it into place. Once in place, you can seal it with GE Silcone I (for windows and doors, not the stuff for bathrooms, which contains a fungus inhibitor, and wouldn't be good for your tank.) Seal both sides of it wherever it touches the glass, both up the sides and along the bottom. You'll need a caulk gun to use the tube of silicone. Don't even bother with the so called "aquarium safe" tooth paste kind of tubes you can get at pet stores. They won't do the job, and you'd need a dozen of them. GE I is cheaper, easier to work with a caulk gun, and is perfectly aquarium safe after a cure of about 76 hours. You just run a bead on either side all the way around wherever the plexiglass touches the glass tank, smoothe it out and shape it with a wet finger if you need to, so it resembles the inside silicone caulking on your tank. You can trim off mistakes or extra stuff with a razor blade. This should give you an impermeable seal between your pond and the terrestrial area.
When heating the plexiglass, you will need to experiment a bit. Put it on a flat surface such as a cookie sheet covered with aluminum foil, then starting out at warm, 180 or so, increase the temperature as you test out the bendability with heavily gloved hands, of course. When it's malleable, bend and fit it into place in your tank. So what if you are off a little bit? Call it art. As long as both sides fit in a comma curve and contact both the bottom and vertical sides of the aquarium glass, work with it from there. You'll have something resembling what you've drawn. You could also bend plexiglass with a heat gun, but I think an oven is better way to heat it equally for an over-all curve.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus