Is there enough nutrition in turtle excrement to feed a plant?
There would be more than enought nutrients in turtle excrement. I have tanks with both turtles and fish, and regularly use the water to water my houseplants. The wastes make a good fertilizer. But you may find that your turtle may eat the plants – turtles like to eat aquatic vegetation – or at least tear them up while swimming through them. Another problem you’ll face is that the plants won’t really help tp keep the water clean. They’ll trap any solids among the stems, and make it that much more difficult to get out if you’re using a siphon to clean the tank.
