I've seen quite a few tutorials on how to build a vivarium (specifically for dart frogs) but I'm confused on the use of the bottom layer of gravel and things like that. Should you fill it with at least some amount of water, or just use it for drainage? I've seen one tutorial that says to fill it with water and it makes sense that doing so would help for humidity and aesthetics if you have a pond of some sort. But I also wonder about standing water. In a lot of vivariums I've seen with ponds (without waterfalls), I see no sign of a water pump or other way of moving water. Is standing water okay? Is tossing in a moss ball all you need to do?