Thank you all for being understanding and clarifying what appeared to be conflicting information when gathered from various sources.
Yes, there are plants in the pond - both buried and floating. Water hyacinth, parrot's feather, floating heart, and something from the colocasi family, I think. I have been draining and refilling it about once a month, and weekly remove debris, algae, add water and conditioners to help break down the turtle waste. Still need snails and some underwater plants, but that can wait for the 'remodel'.
Pets versus wild: these guys are very dear to me - I really do prefer critters to people in most cases!
My intention was to provide as natural of an environment as possible. This was all created for them, then the waterfall idea didn't work out so well. There is a pond pump, but no filter (yet!) - that will happen at the same time as the materials needed to get the waterfall working properly.
I have only had them outside since late spring, and plan to bring them in over winter. I was concerned with it being too cold, overall, but they have all been doing really well, surprisingly. As far as growth and activity, at any rate.
Thank you all, again. I will continue to improve the environment as we go and eventually we'll have a bigger, better, more functional pond that will be shelterable & controllable enough to keep teh turtles in year around.
Hope to see the remodeled pond when you're done.