This water tank is an extra tank that's totally
separate from the fresh water system, right? Not connected to it any way, including the fill....right? If it's yes, just connect the head intake line to the discharge line ahead of the pump. If the discharge from the tank is higher than the toilet, put a loop in the line. If the answer is no, you can't do it. Never mind that you don't use the fresh water system now, you might...and the next owner won't know that he shouldn't. If you want to flush with fresh water, you have 3 choices: 1. repace your toilet with one designed to use pressurized flush water (safe to connect to fresh water system);2. Install a separate flush water tank. Easiest way to do that: stuff an unvented bladder into any location that's convenient to both the toilet and the head sink drain. Tee the bladder fill into the sink drain using a y-valve...connect the head intake line to the bladder. When you need to refill the bladder, open the y-valve to the bladder and run water down the sink.3. Tee the head intake line into the sink drain. To flush the toilet, close th sink drain seacock, fill the sink with water...flush.Or, you could just put a strainer on the head intake to catch whatever is clogging it up.