Elephant Ears are plants they look like Taro plants. They grow from a single rhizome (sp) which then spreads. They can be gorgeous plants, just don't use them in water you want your animals to drink. Cattails, bamboo, lilies are fine and non toxic as are many other plants.
If its 50k gallons, I would get your water tested. You never know whats in your water till you do. Remember water attracts both prey and predators. If its not fenced in, it could become the new local chicken bowl....
As far as the water going stagnate with that size I would doubt it but you could still use something to stir the water or create some type of movement. Moving water is alive, still water dies essentially. (Don't jump on me people that was a generalization) A single water pump float in the middle would suffice I think. It's to put oxygen into the water. I assume you have fish in it. What types? Your birds will love it and I doubt they will drown unless it has a sudden drop in it.
Bubba