I do them all on Sunday and use an Aqueon water changer (python) on the 345 & 40 to do a weekly 40-50% wc and a traditional gravity siphon/gravel vac on the 20 to do a weekly 25%.
If I get extra tine during the week, sometimes I split the 345 into two separate wc's at about 30% a pop.
I change all my tanks every day, whether I am around or not!
I currently have 5 tanks set up at my house - 55 (empty grow-out), 65 hex, 75, 135, and 210.
The 210, 135, and 55 are all connected to a 300G Rubbermaid sump that is in the crawl-space of my house. I have a line tapped into my return pump that has a needle valve attached. The valve lets me adjust the flow from a trickle up to about ten GPH. The outflowing wastewater goes through a DIY heat exchanger that warms the incoming well-water. A float valve in the sump controls the incoming water. In the summer I open it up so that I am changing 100+ gallons/day, which keeps me from having to worry about the temperature getting too high (the 135 and 55 are in the garage). In the winter I adjust down the flow so that I can get by with 1100W of heat for about 500G of water.
The 65 hex and the 75 both have float valves in the tanks and Aqua Lift pumps that suck out about two gallons 6x per day. The pumps are attached to a digital timer, so I can adjust the amount changed at the touch of a button.
All my wastewater goes into my septic tank, but I am considering burying a 600G holding tank in the yard and then watering my grass with the water in the summer.
