I think I would siphon the water out of the tank, not pump it out. That means the tee would go in the canister filter return line as high up near the top of the tank as you can get it. Then through the solenoid valve and out to where you want it to drain to. The problem is that solenoid valves tend to fail in the open position. If that happens, you will be constantly draining the tank, with the canister filter pump pumping water out of the tank. I think the canister filter return flow will drop to near zero as water begins to get lower in the tank, but it may not drop far enough that you won't empty the tank. It depends on the flow vs head characteristics of the canister filter pump.