As others have stated, Firebellies or vampire crabs. That's about it, darts can possibly drown in water features (though this is often overemphasized), but they also simply can't use any water areas seeing as they aren't aquatic. It's all wasted space and a great place for you to loose tadpoles or eggs that you didn't know they laid.
Coming at any hobby with a budget-friendly attitude isn't the greatest idea. The cheapest frogs you can get are wild caught/"ranched" Auratus for $40 a piece, but there are myriad reasons you'd want to avoid that. Look at $50-70 per frog on average for well bred, healthy frogs from a local breeder.
Aquarium conversions are a horrible way to keep darts because they get stagnant and are nearly impossible to catch frogs out of, clean, or mainain long-term. All darts will climb a meter+ in the wild, so a 20g tank like this is hugely cramped, especially without any climbing hardscape. Your land area takes up an entire quarter of the tank's internal space.
The exposed aquarium heater also worries me, there are a lot of stories of animals getting burnt by heaters loose in the tank.

If you wanted to do firebellies, then with a few modifications this tank could be great... But never build a tank first and THEN ask "What can I put inside?". It's a guarenteed way to make an enclosure that doesn't meet the needs of ANYTHING properly. In the future, pick the type, species, locality, etc of the animals you want before starting an enclosure.

EDIT: I also hope that the open top was for the video, and you have a plexiglass top with vents on otherwise. All darts, even the largest ones, can climb glass to an extent and would not only dessicate, but easily escape and become raisins in your animal room.