personally id net the lake if possible and see exactly what is in the lake then id remove all the roach etc and sell to another fisherie etc then if you not to bothered about future breeding of the cap id buy myself a couple of wells catfish about 4lb mark and they will keep the pond in good nick the reason for this is if the pond is full of lil roach etc and yor big carp buy adding a load of perch will up the nitrate, nitrite and the amonia firstly by the extra pressance of fish but if all thses little roach are eaten the perch have to shit them out somewere , i had exactly the same circumsta