[Chin_Lee]

I've had this idea but never had the chance to test it out so I'll post it to see what you DIY guys think.
Since the seawater starts to freeze around -2C, i figured why not use a small chest freezer instead of a bar fridge. I believe that freezers have a more power compressor than a bar fridge so if thats true, then the plan is as follows:

1) Get a small (or whatever size you can accomodate) chest freezer off the buy&sell
2) find a BIGGEST container that can fit inside the freezer to occupy the most space inside the freezer
3) throw in 50 feet or so of 3/8 or 1/2 inch plastic tubing and space them out as much as possible inside the container
4) drill through the freezer wall and plumb plastic tubing through
5) plumb the other ends of the tubing into your sump
6) attach two pumps to one end of the tubing
6a) pump A will be a small pump to keep the water in the tubes moving and will be on constantly to provide a very slow flow through
6b) pump B will be a bigger gph pump on a thermostat to turn on when you want to start cooling
7) fill the container inside the freezer with water
8) add auto antifreeze into the water
9) put a small powerhead into container of water to keep water moving
10) set your freezer to around -5C or so

You will probably have to play around with the freezer settings, temp of antifreeze and the small pump setting until you get the desired effects.
The antifreeze will stop the container of water from turning into block of ice and the powerhead in the container will keep maximize heat transfer from the tubing
During the day when the heat rises, it will transfer the heat into the container of antifreeze through the tubing and the freezer may or may not have to run as much as a bar fridge.
Overnight, the freezer will bring the antifreeze back to -5C.
The small pump will keep the water inside the tubing from reaching below -2C
I have no idea if antifreeze will ever leak through plastic tubing - but I'm sure somebody out there will know. Considering antifreeze is stored in plastic containers thats thinner and softer than the tubing that we normally use, i believe it will be safe.
You can probably even store your frozen fish food in the freezer if its strong enough.

This is just an idea that I've been throwing around for a while so give it a try if you think it will work for you.

CWLee