Some of my fish died during treatment, so that I am wondering if my fish do not tolerate the active ingredient, JBL Furanol?
JBL Furanol contains the active ingredient, nifurpirinol, a tried and tested standard agent against bacterial infections. It is tolerated well by fish. Furanol is not known to kill any fish.
Please note that JBL Furanol is an antibacterial agent, so that it damages the filter bacteria, as is described in the package insert.
Unfortunately, it is not possible to provide further information without more precise information about the situation in your aquarium, e.g. the current water parameters and the parameters prior to treatment, what regular care the fish get, the kind of disease and the severity of the bacterial infection.