A friend of mine has said he was recommended by a RETAILER to use Rabbit droppings for plant food. They are supposed to contain all the trace elements needed for a healthy tank. I understand rabbits eat their droppings after their exit 1st and 2nd times to gain nutrients but on 3rd time they leave them. These are the ones to use as they will contain less ammonia. I have wondered if they are taken from a hutch, they may be best soaked in water as they will have been sprayed with urine. If they are taken from the wild the 3rd time ones are probably lighter(?) He suggested they should be placed every 10cm in the tank, but I have not used them in mine yet I imagine this will generate some interesting replies. I look forward to seeing them.