I put a cinder block in my filter sump on my pond because I heard from the Phenoix koi rescue site that if you have a full PVC filter system your pond can have a ph crash. It's been about a week and a half and my water has gone from thin green to a transparent brown type like when you add the Tetra black water extract to your discus tank. I am happy that I can actually see the fish now but I am worried what type of affect its going to have on my fish. A local koi supplier I asked said that cinder blocks leech lime into the water which is "not so good'' for the fish. I haven't checked my ph on that pond ever so I wouldn't know what type of swing in ph. Gotta try test it this weekend hopefully. Does anyone know more about this and which one is true?