Tench Diseases

Does anyone out there have a clue what could be the problem with our Tench. suddenly last week over two or three days we found a number of tench dead, floating, wih no visible signs except a red mark on the underside of the fish. All were between 4 and 8 lb, seemingly healty fish - no eye or scale discoloration - just this red mark. Has anyone come across this beofre?, no other fish are affected - we have carp roach rud sturgeon and some black bass mainly in the lake. any ideas would be appreciated.

Thanks
MAT79