I really like the idea and I love you tank! I agree that the health and size of the tads is likely due to a massive amount of natural easy to find food. I can only imagine how many little creatures you would find in a sample under microscope. When I kept a lot of fish I always loved to examine the water to see how well the micro environment was doing. It could be a matter of there being a lot of small animal life that they have been eating and growing better than an all algae or other diet.

Do you dose fertilizers for the plants? I wonder if this is also helping their health and size because they may be getting minerals and such from the water.