You can get a small pump for table top fountains for under $10 at Harbor Freight.

Actually, you could set the whole thing up like an aquarium. An air pump and under gravel aquarium filter would work and look good. If you can hide it(I am guessing this is a decorative project)an old fashioned air pump and gravity filter for aquariums would be adequate. They size them for various gallons, as you know.

The difference is that pond pumps need to move a lot of water containing a lot of debris and organic material through or to a comparatively huge filter. The air pump may just blow air but it carries water along with it and circulates and aerates it just fine for 20 gallons.

Unless you want to incorporate a fountain or waterfall in your little indoor pond you don't really need a water pump. A bubbler (air) pump will work just fine. After all what you will have is still an aquarium. It just has a different container.

You do not need an aerator or whatever for a small pool that houses plants and a small quantity of fish. I sank a small half barrel liner into the ground in my front yard and put in some river sand and then planted it with a Sagittaria species and added some rosy red minnows that I got at the local Petsmart. I had Parrots feather growing nearby and I introduced some water hyacinth. Needless to say the water hyacinth has taken over the pool and I will have to get out there, in my front yard, and clean them back. Also the parrots feather has found the pool. The pool gets prob. about 6 to 8 or more hours of direct sun, that is once I remove all the covering plants!!! There is no aeration or filtration. I have a similar pool in my backyard but not sank into the ground in which there is a Potamogeton species and White Cloud Mountain Minnows and again no aeration or filtration but the raccoons sure do mess this one up trying to snag the little fish that they can feel but cannot grab hold of!!!!
Here is the front pool before weeding out

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Here is the pool after weeding

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thank-you to all for the very useful comments,my indoor pond is still up and running,no stagnent water,still using aerator only,plants rooting and i let my comets have short vacations in there,they absoulately love it,cant make it a permanent move for them as im worried about scale damage due to the rough surface of pond,maybe i will drain/put in pond liner[any comments welcome on this)or drain/move to garden for wildlife(no raccoons here just cats and squirrels(uk).
once again thanks for the help and positive feedback.i will post some pics when i get a decent camera.