I haven't built a multi-res chiller. The idea is from a similar project that could easily be adapted. The original was a chest freezer adapted as a single res chiller.
I'll give a quick run down and if somebody actually wants to go build one, I'll help you through it:
The chest freezer is partially filled with glycol (non-toxic marine antifreeze), or a large container filled with glycol is placed in the freezer.
Stainless steel tubing is ran through the wall and sealed. This is coiled into a heat exchanger and submerged in the glycol. For a multi-res, you would use multiple coils.
Each res would have it's own pump and coil and an ETC (electronic temperature controller), which can read the temp in each res and turn on/off the pump as necessary for cooling. The freezer (chiller) would run constantly.
If you only have a couple small reservoirs, it may be more economical to just go buy a couple chillers, but small chillers won't handle big res's under HID lighting in tight boxes. A small freezer should hold up to about anything you can throw at it...
Anybody wanna try it?
but we got our mother now so were cool.