I have used this cookie jar a number of times as an aquarium of sorts, scooping up a bunch of pond muck and watching all the critters that live in it grow, but I finally decided to make a proper tank of it recently, here is the result.
It's a 3 gallon glass jar, approximately 16" high by 12" diameter. It is warm enough now that I was able to remove my heater; I don't plan on replacing that, as the guppies won't be in there when it gets cool again. I've got an aquaclear 10 powerhead in there right now with a sponge on the intake so that there is a current in the jar.
For lighting I drilled out the inner 1 1/8" of a 1 1/4" aluminum rod for a heat sink and mounted a 6500K 3 LED star with Cree XP-Gs inside the recess. I picked this particular configuration because it runs at 9v and I happened to have a 9v 1A transformer lying around. When I tested the transformer (after ordering, of course) I found out that the transformer was actually putting out 12.5V. Bought a 'universal' type AC-DC transformer and set it to 9V, 600mA and that is doing a great job of powering the light. The initial heat sink was inadequate so I added a small finned heat sink that I had to the back of it. The combination works well, especially in the humid environment that I've got it, though the light is a bit closer to the water than I care for now. I was going to paint the LED/star with polyurethane to seal it against the water it was sure to encounter, but from another thread on here I got the idea to paint it with clear fingernail polish. Found out the hard way that the waterproofing worked when I dropped it into the water while it was turned on.
The tank also requires an airstone, both for cooling the light and for aeration. With the close fitting lid on and the light running, there wouldn't be any good way for fresh air to get to the water, except through the small notch I cut in the edge of the lid for wires/tubes. I'm also using DIY CO2 on this tank, hopefully I'll be upgrading that to canned in the future.
Ultimately it will hold only RCS and snails, but at the moment there are also a bunch of guppy fry in there as well, and a few ghost shrimp. As far as plants, I've got ludwigia repens, something that was labeled as dwarf hairgrass (though who knows?) and a mystery plant. The driftwood is the root bulb of a forsythia bush that was uprooted about a year ago. I boiled it for several hours then kept it in an 8 gallon cooler that I filled with boiling water each day for 2 weeks before it stopped leaching.
The substrate is soil that I dug out of the creek in my back yard. (No fish or plants were disturbed in the digging of the dirt.) I allowed it to soak for a week or so, mixing it regularly. I then dried it, pulverized it into dust, then baked the dust to kill off any cysts/spores that may have taken the ride. Anything that can survive 2 hours in a 350 degree oven deserves to infest my fish tank. I used 1/2" of that topped with 3/4" of small gravel (3 - 5mm size.)
The picture on this page isn't the best, because of the positioning of the tank and myself (it's on my computer desk) I cannot see any cords, hoses, pumps, or filter sponge when I look at it, just the tank and some bubbles coming up in the back. The driftwood also looks much cooler. I'll try and get a better shot in the near future.