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This is where you get into the realm called "pico." Pico tanks are exceptionally small nano tanks. They are generally skimmerless, either relying on a refugium/sump or those old HOB filters, usually modded out to meet the needs of the tank (in the case of that small, you could definitely do a HOB refugium using modded out Aquaclear)
The trick is, you have to be EXCEPTIONALLY strict on yourself. Going pico requires more human effort than larger tanks, or even the slightly larger nanos. They generally require topping off throughout the day, as well as weekly to bi-weekly water changes. It also requires VERY limited stocking for your tank. In a five, a sexxy shrimp could be fun, or perhaps a pom-pom (boxer) crab would be interesting (obviously along with coral!).
If you have any questions, don't hesitate to ask! I adore pico tanks!
ready for this... my first reef tank (first real sw tank for that matter) was a 5g hex marineland deal. Small heater, circulation was through the "filter" that is built in, screw in coralife 50/50 bulb and rockwork almost to the very top (will have to see if I still have a photo). I kept a green banded goby (great personality in a tiny fish), and some hermits & snails... zoas, shrooms, frogspawn. Because of the sealed up top I had very little evap issues. Tank was doing wonderfully, only took it down because I "upgraded" to a 24g aquapod... actually that little goby was the only fish in the 24g aquapod who survived the move to NJ and he's still going strong.
I'm actually thinking about setting up another 5 (or maybe even a 3) on my desk with a green clown goby and a couple sexy shrimp.
You ought to see PanMan's 3/4g (yes, as in three quarter gallon!) tank. It's really cool but I don't envy him having to do those shotglass sized water changes.