I've had a couple requests to further explain what I was talking about in prior threads, and even though this idea isn't particular unique or new in any real way, I figured it would still be helpful to show others what I've been talking about.
Here we see a Coralife Tee connector, it's technically called the "Coralife Turbo-Twist Hose Locking 5/8" T Connector" and can be bought at Big Al's for $3.49, part #7932. You'll also see a "Two Littles Fishes" ball valve, part #23492 at Big Al's. The clear tubing is ordinary hardware store stuff, and the green is coming from my Eheim 2028. All of this is on the outflow side.
When I want to drain the tank I open the ball valve here, the filter will then push water more or less half through the clear tubing and half back into the aquarium. Another ball valve on the Eheim tubing would allow one to divert water fully, but I'm not in that much of a hurry.
Here we see my dirty bathroom sink, but more importantly the other end of the clear hardware store hose. There's the standard Python venturi, another TLF ball valve, and a 'spare parts' Python hose connector.
To drain water, this ball valve is also opened, along with opening up the Python Venturi. When it comes to refilling, with that venturi opened, but the ball valve shut, I can run water through the venturi checking for temp with my instant thermometer. Whenever water temp is just right, I can then open the ball valve and shut the venturi to send water through the hose.
Here's another shot of the Eheim outflow setup. Prior to doing the above, where I send water back in, I pull back on the Eheim's double tap connector setup, so as to shut water flow off. I then flip the switch to turn the filter off too. This ensures that the water isn't fighting with the filter, and that no untreated tap water enters the filter either. My inline heater is 'on the way' from this setup to the next photo. I, also, of course, put my 2 capfuls of Prime into the aquarium.
Here we go, while I go make my sandwich for lunch, water flows back into the aquarium, passing through the inline heater just in case the water temp dropped a little on the way.
Once the tank is refilled, I close the ball valves, coil the clear hose up and set it behind the aquarium, and turn the 2028 back on.