I built this the other week. Works quite well so I thought I'd share it.

It's your basic PVC overflow tube with a hose attachment in place of the usual drain T.

Here it is in action. The black hose is the drain going out to the flower bed. The gray hose is the fill hose going to the suction cup fill nozzle seen to the right. Quite a lot of tannins in this tank.

I left the fill hose on a very slow trickle for a good span of time. Having the fill and drain hoses so close together meant that some of the fresh water flowed back out the drain after mixing a bit. But that was part of the plan. I wanted to minimize any shock to the tank by allowing the water to change very slowly.

One improvement I definitely want to make before using it again is to add carbon filters to the fill line to make sure any chlorine is gone. We did still get a bit of shock and I'm not sure whether it was the water chemistry or the temperature which caused the problem. An ordinary rack of undersink filters should clean the water nicely.