Based on what betta4me is talking about, as well as my own experience with a 10 gallon tank at home, you might not have to limit yourself to cold water fish.
I started a recent discussion here
http://www.aquariumadvice.com/forums/f12/unexpected-heat-source-109545.html where I pointed out that I apparently have something heating my fish tank. I was letting water for PWC sit to warm to room temperature, but when I would go to use it, I noticed that the water in the fish tank was always warmer. Now my 10 gallon tank at home has both an in-the-tank filter and an air stone. But as a group, we ruled out the extra heat coming from either the filter or the air pump, and that it must be coming from the light source.
Basically, if you have a tank dedicated light, it can raise the temperature of the water by as much as 5 degrees (depending upon how long it's left on every day.
However, you're left with the issue of the filter. As someone has already pointed out, if you use the filter system that comes with an Eclipse system, it has to run 24/7, otherwise the biowheel will dry out and kill the needed "good" bacteria. If you keep the bioload VERY LOW, you can follow happygirl65's advice, which is to basically get lots of surface area inside the tank to allow "good" bacteria to grow on surfaces rather than in a filter. What you want is stuff with LOTS of nooks and cranies to have LOTS of surface area (i.e. you do NOT want smooth decorations). Plants are also good because they will have lots of surface area relative to their volume.
From there, you just need SMALL fish so that you won't have a large bio-load on the "surface filter". So perhaps tetra's would work real well for you. In the case of Cardinal Tetra's, they might need temperatures from 73 to 81... perhaps a bit iffy if your office stays below 70. But there are others like the Neon tetras that range from 68 to 74 degrees. (Note, temp ranges base on what I could find at
www.liveaquaria.com).
So if you want a tank limited to a single plug, then I would suggest you use that plug for a light, get lots of stuff inside the tank, and then limit yourself to about 3 or 4 tetra's.
Otherwise, if you try to go with any fish larger than tetras, I think you would have to limit yourself to either a single fish or dedicate your single power source to the filter, and you would loose the light, discount the possibility of live plants.