Thanks BadOleRoss for the info, but I did away with using the THWH a while back in the thread. I'll be feeding the PEX from my existing hot water heater by putting a Y on the faucet that supplies my washing machine in the laundry room right next to the fish room.
Egon, I did post pics after. Check around post 616. The PEX will be wrapped in pipe insulation from the faucet to where it enters the tank. I shouldn't have much heat loss in the 15' run from the faucet to the tank. BTW, the hot water travels through the PEX and never enters the tank.
The PEX is in the tank, a thermostat turns on the second thermostat that checks the temperature at the end of the PEX run and opens a solenoid valve that allows hot water to flow through the PEX until it's hot enough. A pump that is always on pumps water through the pvc frame the PEX is mounted on to circulate the heat throughout the tank.
Just imagine the heater coil inside a regular aquarium heater. The only difference is that instead of electricity flowing through the coil and it being made of wires, mine is made of PEX and hot water flows through it.