If you go low-tech planted FW tank, I think you'll be happy. Plants suck up a lot of the extra nutrients and act as a natural filtration system.
Put a thin layer of gravel/substrate, add some larger rocks, and cover the bottom with Java moss and Java fern. These plants don't need much light. Throw in some floating plants to shade the tank from the lights. Dose lightly with iron-enriched plant fertilizer at water changes, and you're set.
Oh yeah, throw some snails in there to munch on the algae and detrius. They work great.
I'm planning to get a bigger FW tank at some point, maybe 40 gallons or so. I've got a Paradise Fish, so I don't need a heater. I skipped the mechanical filtration part, too.
Here's an older pic of my current 9 gallon tank:
There are two floating plants in the picture: Salvinia natans and Water Lettuce. I've since removed the Water Lettuce, as it grew in too much. The Java moss and fern at the bottom have grown in a lot since this picture was taken.
Like I said, I've got no heater or filter. There's just one 27W daylight spiral bulb. Very low-tech.
My 2 gallon Betta tank is quite similar, with only a 9W bulb lighting the tank:
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