Grass carp tapeworm disease, caused by Bothriocephalus acheilognathi living in the intestinal tract, has quietly become one of the nastiest problems in freshwater aquaculture. Farmers who have been through an outbreak know exactly what it looks like. The fish turn dark along the back. They hang around the pond edge. They stop eating for days. And then they start dying — even though, from the outside, nothing looks obviously wrong. No red belly, no fin rot, no gill damage. Everything clean. But the fish keep wasting away. The speed of this disease catches a lot of people off guard. Infection sets in during the early fry stage, and within a short time…