A 125L works great for cichlids; what species you plan to keep? Don't remember exactly the height of a 125L (20-21 in.?) but think it will be OK to keep White's in and based on a 20 gal. per frog requirement you could keep lots of them in there. Just remember to quarantine any newcomers before releasing in a large enclosure with healthy frogs.
The
Frog Forum - White's Tree Frog Care - Litoria caerulea discuss a false bottom and if by any chance you have an aquarium under gravel filter that size; they work fine and cut down on the build up time. Only thing you have to do is get plastic caps to cover the clear tube risers at a hardware store (furniture leg tips). The tubes provide access to false botton and it's easy to insert flexible clear plastic tubing down the filter tube to drain any collected fluids if required.
Looking forward to seeing pics of your build process and good luck

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