Darts would be very uncomfortable on an all gravel substrate, and there would be nothing to break down the feces, so it would get very unsanitary rather soon, and you wouldn't have a biological recycling situation. At least put a layer of weed cloth or fine screening on top of a gravel drainage area, and add a layer of coconut fiber or light brown sphagnum on top of this, or both, and some oak or magnolia leaf litter if either are available in your area. You won't have to change it, only add a top dressing of substrate every year or so and perhaps drain off any excess drainage water. (You may not even have to do this, if you have plants.) The pothos will do just fine if you just stick a cutting of it in this and keep everything misted so the soil stays damp and the humidity is about 80%, which you would have to do for the frogs in any event. The roots will find their way through the substrate and even into the gravel eventually. If you do this, you won't have to continually disrupt the frogs by having to clean the poopy gravel, if the frogs even last long on it. Except for trimming excess growth, if you stick with the simpler small tropical plants and vines, plant care is truly minimal. Ficus pumila sp., Pellionia, (water melon vine) most Pilea, Fittonia, Peperomia sp. are virtually care-free and don't require a green thumb.
Honestly, if you stick with the more simple, easy tropical plants and small vines, they are not at all a problem to grow, and will be healthier for your frogs, without much maintenance at all.
Cross my heart, hope to die, if I told a dirty lie!
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Patty
Pahsimeroi, Idaho
D. auratus blue
D. galactonotus pumpkin orange splash back
D. imitator
D. leucomelas
D. pumilio Bastimentos
D. fantasticus
P. terribilis mint and organe
D. reticulatus
D. castaneoticus
D. azureus
P vittatus
P. lugubris