Many fish will eat them -- I had a leopard wrasse for several years in my 240 and never saw any flatworms. When the leopard wrasse died, I experienced a flatworm explosion almost immediately after his death, and I hadn't added anything new in a long time. I am pretty sure the leopard wrasse (which is a natural predator of flatworms) was keeping the population in check 24/7, and when he was gone, there was nothing keeping them from reproducing like mad.