The problem is you leave the valves open! The system is not designed to do that, you get build up and it clogs the pipe. To try and fix it you need to open the hot water faucets and hope it is enough to dissolve the build up! Both tanks, black and grey are called "holding" tanks. They are called that for a reason, they hold the contents, and then are drained. This action "sweeps" out any build up and keeps them clean! If you are able to get the tank clean, please do yourself a favor, and keep the valves closed, until the tank is full, then drain! Simple concept, why do so many people think they know more than the engineers who designed and built these systems?