Yeah, I only use those for grow-outs. As FG said, by the time substrate goes in, they are pretty ugly tanks. Now if you can find a way to make it into a vertical conversion, that might be an interesting tank
I like 18 x 18 x 24 ExoTerras for a pair of thumbs (R. imitator are my favorite recommendation for a first thumbnail frog, though any Dendrobates species would be much more forgiving of novices getting the hang of care).
I've kept pairs of thumbs in 12 x 12 x 18 ExoTerras, but it isn't a great option -- there just isn't enough space in 1.5 cubic feet to give the frogs a range of moisture, temperature, lighting, and hiding options.
Edit to add: or InSitu vivs (I have two Amazonias, which are just a little larger than the 18 x 18 x 24 Exo). ExoTerras are extremely hard to come by these days, but AFAIK InSitu is still shipping as usual.
Problem with fish tanks used for darts is air movement and circulation, specifically getting circulation down lower in the tank. You can make it work, but you are going to have to be creative and work for it. Vents down low and vents up high facilitate better air exchange and turnover. You can make it work though with that type of tank you may be more limited in what you are able to successfully keep plantwise
I have two fish tank vivs (well, one is a viv, one is plants only) and they both, frankly, suck. I sure wouldn't wish either of them on a novice who has enough of a learning curve to conquer without creative housing concerns.
Access from the top is a pain compared to front opening vivs -- as someone who has had dozens of aquariums over the decades, I admit that it is hard to understand why this is no problem for an aquarium but it is a problem for a viv, but understand it I did, eventually -- and this top access is only a benefit with those few species who have a tendency to jump out of open doors (none of which are really beginner frogs). Accessing from the top is made even more difficult by the fact that the lighting has to be on top; I solved this in my viv by suspending an AI Prime, but that is an extravagant toy for a vivarium (a $200 light that grows plants about as well as a $20 one).
Ventilation in a fish tank is very poor (by design, right? ) unless you drill vents into it near the bottom. I did that to one of mine (the viv), and it did help venting substantially. The other has a circulation fan, which simply doesn't work nearly as well as passive ventilation, which is more gentle and even. Some people do get fans to work, but after the expense of the hardware and mounting solutions I don't think it is money saved, and fans and power supplies need periodic replacement. Passive venting is free, and continues to work even after the cat chews the cords.
The dimensions on many fish tanks -- including a 20H -- are bad for landscaping; there isn't enough front to back depth to make the 'jungle gym' that frogs really get a lot of use out of. That's the reason vivs tend to be cubes, or nearly so.