well i think the rabbit poo thingie would work wonders for your garden, but in a planted tank, i think it would have to be prepared.. you see any kind of poo contains nutrients that plants could use

but when it comes to rabbit poo, simply placing it in an aquarium might lead to er... stinky water....
additives to a planted tank come in 3 forms only i think
initial nutrient rich substrates (flourite, floredepot)
liquid fertilizers (like sera florena ? florapride? i forgot)
solid fertilizers (like root tabs)
other forms of additives are c02 injections etc etc
hope this helps