I agree with this advice.
OP: welcome aboard.
Broadly speaking, water features, such as this paludarium, are not really appropriate for dart frogs. The "usable" land space is smaller than the 18x18" footprint because of the space for access to the water section.
Additionally, the water section is so deep that is a death trap for any fruit that happens to fall into it.
Personally, I wouldn't use this setup with any dart frog species.
Thank you! Yes, looking it over it does not have very much truly usable horizontal space on it as it currently is set up. I do intend to add more wood "roots" that extend into the water, as well as water plants such as small lilies so that regardless of the occupant type I wind up with, there will be plenty of places and opportunities to extricate themselves from the water. I was leaning towards darts but am definitely open to other species, particularly if other species would be better suited.
That enclosure looks awesome!
You'll find paludariums are rather frowned on as an enclosure for dart frogs here, and with good reasons; the risk of drowning, difficulty with humidity fluctuations, less livable terrestrial space, pathogen transmission, among others. I've seen/heard of a few that worked out, but those were all purpose built from the ground up for PDFs and by those with a wealth of experience. This enclosure itself would be even more difficult to adapt to darts. There are many ways the frogs could get stuck in the (very deep) water and not be able to get out (even with the addition of one ramp). There is also little space on the ground above to add leaf litter, and with the slope much of it may end up in the water anyway.
It's likely with some serious modifications it could be adapted, but by the time you were done much of the visual aspect that makes this enclosure so stunning would be lost, and to be honest that would be a shame because it looks great.
Hopefully more people with chime in with their own opinions here, but If you are determined to stick with frogs here I think you will be better off with tree frogs. Even with tree frogs I'd be a little concerned with the risk of drowning though as the way it was built makes getting in/out of the water difficult for most critters. (Maybe you could add or incorporate some type of "roots" growing out of the bank and into the water on one side to provide another escape route and add more visual features to the water section?).
If you were looking for visually appealing tree frogs I'd probably start by researching Red Eyed Tree Frogs, Tiger Leg Monkey Tree Frogs, or Reticulated Glass Frogs.
Yes I am definitely finding this

I have seen several beautiful dart set ups that involve a water feature, but even those were MUCH shallower than this one. So I am certainly very inclined to agree that this set up is really not well suited to darts now that I have looked further into it. I absolutely ADORE RETFs so that is a distinct possibility. I haven't ever really looked at the other two you listed though so I will certainly look them up!
The only frogs I’ve heard of that do well in paladariums are mossy frogs, which are definitely worth looking into
Ohhhhh I have seen and heard of these but that had not even occurred to me! Definitely will be looking into those. From what I understand they can be difficult to come by.
Fire Bellied Toads do well too!
Neat! Will also look into these!
In my opinion, water features tend to be for aesthetics only and often do more harm than good. If there is any way to drain the water and convert the bottom into usable space, you'd have quite a nice looking vivarium at that point. If you go that route, a group of leucs would do well in there. Very vocal and would make good use of the vertical space.
Hmm.. Well I don't have any intention of removing the water feature, but am totally ok with going with a different species that would be better suited to how it's situated
Completely wrong build for a dart frog.
90% of all new hobbyists want them. They don't understand that Dart Frogs won't even "use" / need / enjoy it. Large water features are solely for the owner and not the frogs. Some considerations:
NEGATIVES:
1.It will cost an extra $70.00 (avg)
2.the water will discolour and turn brown from tannins and substrate leaking into it
3.the water will discolour due to excess supplement powder
4.the water will discolour due to dead fruit flies
5.the frogs will not be able to eat a portion of their food (fruit flies) due to them drowning in the water
6.the frogs will not be able to ingest some of their supplements due to the water / flies in the water
7.waterfalls are hard to place and scape in.
8.waterfalls and pumps, once hardscaped in, incorrectly, become a bitch to remove or adjust
9.the frogs will have that much less living space and room to move around due to the water feature
10.the water will "have a mind of it's own" and will suddenly change or go where it wants, F'ing everything up
11.water will soak your substrate
12.water will rot and kill your plants
13.the pump will be more noisy than you originally expected
14. a waterfall or large water feature will look goofy and cramped in a little tank (under 50-70 gallons)
15. It may turn out to look like a papier-mâché lump or concrete blob or that f'ed up volcano you had to build in 6th grade.
16. Constant wetness will give your frogs a bacterial infection.
The Positives:
1.If done right...it just may look "cool". Well, at least for the first 60-90 days, after which it will 'not' look cool due to one or more reasons above.
2.it will add some additional humidity
I see, well, I wouldn't really consider myself a new hobbiest lol. I do only have one frog, but have been keeping her for well over a year, and have been keeping a variety of other exotics and aquariums for many years. This isn't my first big enclosure, and I have built many other vivariums from the ground up, with and without water features. Nearly all of the negatives you listed wouldn't really apply here. Pump sounds don't bother me, and if 70 extra dollars was going to be a deal breaker, exotics wouldn't be the proper hobby for me anyways LOL (but either way, the habitat is already built and bought). But also the water issues you listed for example. This paludarium has a full bacterial filtration system going. Meaning that the water is balanced, and tannins are actually beneficial to that. Nothing wrong with a little tea colored water

In fact you can already see the tannins in the photo. I honestly like the look, personally. I also intend to keep small fish in that lower level, which will readily take care of any stray flies (or whatever feeder) falls into the water and keep the nitrogen cycle going. I will also be adding many more aquatic plants which will help with both water clarity and also provide additional "walkable" surface area on the surface of the water which reduces drowned feeders and allows spaces for any inhabitant to climb out should they take an unintentional tumble. For most of the rest, this paludarium is already built. So the pain of construction is in the past, and it is fully established and has been running for several months. It looks how it looks, and will actually look even better with age I think as the moss really grows in. Do you feel like it looks goofy and cramped? Or like a f'ed up 6th grade volcano? I'm not sure how to take that. I personally think it looks really nice. Which, I suppose is what matters since it will be in my house LOL. I didn't build this one so, I guess no skin off my nose if you think it looks awful haha.
Regardless though, it is looking like other species would be better suited to this particular build, and have been given several good ideas for what to look into, which is exactly what I came here hoping for! I certainly didn't necessarily have my heart set on putting darts in it. Obviously the primary objective is to make sure that whatever inhabitant winds up in there is able to live it's very best life. And if that's not possible for a dart frog then I will most certainly not be putting dart frogs in there. The welfare of my animals will always be my absolute top priority.
Awhile back someone was selling some Granulosa Glass Frogs here on DB. Ad sat for awhile. Maybe they still have them?
Just another one I forgot to mention.
If you got a sexed pair the male should call, they look awesome IMO, live near streams/water, they can climb super well (important with water feature), and most will readily accept fruit flies.
Will definitely look into glass frogs as well! Thank you!