Im not opposed to a pair of fish.
The tank is a 20x20x20 cube. Closer to 40 gallons i think.
20 inch cube is closer to 30 gallons (unless you have a sump)
I have a 18 inch sumpless cube and 9 fish in it now - 4 pairs and on single because his partner died in QT. That might be too much for long term when they are all grown up but for now, while they are all still small, I feel it is OK.
My SPS are largely growing, some actually fast.
If you want something that doesn't need feeding or just minimal food additions you should pick something like mandarins, pipefish, sixlines,...
Many fish can be paired up fairly easy:
Dottyback (the fridmani like): Start either with two small captive bred, but make sure one is a bit larger than the other, or build a pair from larger by selecting a large stretched looking and a smaller that looks in proportion shorter. If going with fridmani you can sex them by their tail fins: the female's tail fin has a vertical end and rounded edges, the male's tail fin has a sloped end and the bottom edge is elongated.
With sixlines I always had success by taking a small, short looking and a significantly larger that looks more slender.
In general, if not an established pair, give the smaller (most of the time the female) a head start of a day or two before adding the larger.
Many other fish can be paired in a similar way.
