I was discussing this with someone on an online forum, and am uploading photos with explanation here for future reference.
I have 20 or so smallish-sized tanks (the largest are 100 liters or so) each equipped with an overflow pipe. A few use a single 1/8″ rigid tube poked through a rubber stopper, but most have pvc overflows like this one:
When tanks are filled above the overflow line, water spills into the pipes and drains via flexible tubing into a holding tank (actually a plastic storage box from Target) that’s a few inches off the floor:
That sump drains via garden hose into a proper sump (the kind designed for flooded basements) that is sunk below floor level. The slab in my basement is 100 years old and has largely turned to dust, so burying the sump in the floor was no big deal.
That sump contains an actual sump pump (again, the kind for flooding basements) which pumps the water outside into my garden in the summertime, and into a laundry sink in the winter. All of this involves high-tech garden-hoses and garden-hose valves available at any hardware store. It is slightly failure-prone due to kinks in the hose… I should probably replace everything with rigid pipe someday.
OK, so that’s how old water gets out of the tanks and out of the house.
New water gets into the tanks entirely via gravity. An ice-maker-style plastic tube is tapped into a cold-water line (the work of 5 minutes or so) and runs into a 30″ x 18″ x 12″ glass tank (the ‘temper tank’) that’s pressed up as high as I can get it against my ceiling. The inlet line uses a float valve to keep from overflowing the tank, and has a manual shut-off down where I can reach it. The temper tank itself has a drain close to the bottom with its own shut-off. In this photo you can see the inlet line (blue) and the drainage valve (red):
Because my city water is chock-full of chloramines, I have to be very careful to have the red valve shut when the blue valve is open… if both are open then toxic water runs straight into the aquariums, albeit slowly. Once the temper tank is full of chloramine-laden water, I need to add a water conditioner to detoxify. The temper tank is way too high up for me to reach, so I use an aqua-lifter pump to pump Prime from a bottle down at arm-level up into the temper tank:
When the aqua-lifter isn’t pumping Prime into the temper tank, it’s pumping air into it, which keeps the water aerated. Water comes out of my tap starved of CO2 (a side effect of something at the water-treatment plant) so aerating it a bit stabilizes the pH. There’s also a high-wattage titanium heater in the temper tank, one with a shutoff so it doesn’t run amok when the tank is dry. That’s useful in the winter when my tap water is icy cold.
After the new water has had some time to warm up and circulate and dechlorinate, I close the blue valve, open the red valve, and the water drains into horizontal 1″ PVC pipes that run along the top of each rack of aquariums. Each rack has a individual shutoff in case I want to change water selectively:
Tiny plastic valves are drilled into the PVC and can be opened or closed (or partially opened) to control the rate of water change per tank. Water drains through the plastic valves and down 1/8″ plastic tubing into the inhabited tanks.
So, when I want to do a water change, here are the steps:
1) Close the red outlet valve
2) Open the blue inlet valve
3) Hold a bottle of Prime over the aqua-lifter tube until a few cc’s are slurped up (the aqua-lifter inlet tube is marked so I can measure how much conditioner has been taken up.)
4) — Wait an our or two —
5) Close the blue inlet valve
6) Open the red outlet valve
It’s pretty painless. It would be much /more/ painless if I had a much bigger temper tank… As it is, a single cycle of the above only does about a 5% room-wide waterchange, so I have to twiddle the valves fairly frequently.
Even better would be if I had a source of non-chlorinated water (or even just non-chloramined water), in which case the red valve would be completely unneeded. I have yet to discover any automated technology for removing chloramines from water.