Here in York PA glass shops charge 10.00 usd per cut. So, when you find 3/16" glass ie, windows, table tops, store fronts, auctions, yard sales, large item pick up days, 2 cuts equal 20.00 usd. 1/8" glass will sag and isn't sturdy.
Another thought using window glass, instead of having the traditional one split in the glass top the length of the tank and having the rear piece stationery and the front piece for access. Think about using window glass with the pieces of glass hinged from front of tank to rear of tank. The plastic hinges can be purchased at pet shop or hardware store. Do you have a building restoration warehouse open to the public. I buy glass there dirt cheap. Cuts would only have to be made along the bow front. At the rear edge you could use that plastic that slips over the glass and can be cut to allow for heater filter whatever. Knobs can be siliconed or gorilla glued in place.