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Dart frog diets in captivity should include supplementation with vitamin D3 for proper uptake of calcium and related biological functions. Vitamin D3 is otherwise produced in the skin during exposure to ultraviolet B / UVB light. However UVB radiation/ light is dangerous. Think of humans and sunburn and skin cancer risks sunlight brings.
Amphibians such as dart frogs require moisture in a humid environment to prevent desiccation and death. Keepers that introduce UVB lights must remove a section of glass to allow UVB to reach the frogs as glass blocks UVB light. Reducing glass covering the top of a vivarium reduces the humidity a vivarium can hold. Ideally most dart frogs should be kept in the mid 80% to 95% humidity range (with gradient ideally). Aside from the lower humidity brought by opening a vivariums canopy, a uvb light fixture will generate heat further lowering humidity. Any uvb touching a frogs skin will even further reduce moisture at the skin level. Excessive duration of UVB exposure increases chance of burns associated to UVB exposure.
Aside from the physical risks associated with use of UVB exposure in-situ there is little research showing that dart frogs bask to receive UVB light. In the rainforest the density of the canopy restricts sunlight from reaching the base of the rainforest floor where dart frogs mainly dwell. Some research shows that atleast 2 species of dart frogs actually avoid exposure to UVB light. "Using field experiments and field observations, two species of poison-dart frogs in Costa Rica (Dendrobates pumilio, D. auratus) consistently preferred areas in the field and within experimental testing chambers that offered low levels of ultraviolet-B radiation. In field observations, vocalizing D. pumilio were found at locations with significantly lower levels of ambient ultraviolet-B compared to random locations throughout their natural habitat.
Thus we do not recommend use of UVB lighting for dart frogs.