I agree that the usual suggestions for a 5.5 are conservative and truly unimaginative.
They are usually being given by people who scaled up to 55's first thing when they found out how little they could keep in a ten, and are now spreading the pain.... :Angel: ...okay, just kidding!
But seriously, until you have kept a small tank, it's hard to give real advice. And very few people have seriously kept a 5.5... I mean really worked at it vs. just had one that got them into the hobby before they quickly moved up to something bigger. But as someone else said if this is your first aquarium, either be prepared to spend more than you thought you would and study like crazy, or just get a beta.
First off let's not pretend that the 1/gallon rule was created after the advent of super filtration systems and the modern understanding of aquarium chemistry and such, as oppose to long before (I mean, the 1"/gallon rule is a throw back to pre-filter days. It is what a system can support with just the nitrogen cycle from bacteria on gravel or decorations, with few water changes and no filter... look it up). The reason you can't keep a whole heap of fish usually has more to do with their need for adequate territory and swimming space... cause you can buy enough external filtration to pack a tank gill to gill if your willing to do a lot of water changes but that alone won’t mean healthy fish... and stress = disease.
I currently run a heavily planted and just a bit "over stocked" 5.5, (but I used to have a 55 freshwater about 15 years ago). I was very careful, as another poster said, to go slow and listen to the messages the water chemistry and the fish were giving me. I also read extensively both about what the textbooks say each fish needed to be healthy and other's actual experience with them.. and in the end I found a good balance, but at first of course, I made some mistakes. All along though, ammonia/nitrite levels have been the least of my problems... after the initial cycle they’ve stayed a 0 ppms.
People talk a lot about a species’ need to school in groups of at least 6, but also consider whether these fish will encounter any other threatening fish in your aquarium. I mean, fish don't school because they have the mental capacity to "be friends" or because they "get lonely" as people put it on many aquarium forums. They school because it reduces stress, ie to protect themselves from other larger fish (a big school can look as intimidating as being a big fish). But if you are the biggest guy in the pond, you may feel a whole lot less a need for a lot of company. In fact, when I first started my tank after it cycled, I put in six black neons (that and one mystery snail were the only inhabitants). I was thinking a 5.5 was a lovely opportunity to limit myself to a species tank. However, they quickly went from schooling to dividing up territory, and one guy became dominate and hounded two of the others to death. He was still at it with the remaining three when I added one of my current guppies. But after the male guppy started chasing everyone (as guppies will)... *Tada* everyone loved each other and huddled in a group again (I decided that black neons were not happy in such a small tank and returned them to the LFS, but others have told me that the smaller regular neons can do just fine even in smaller schools of three to five *if* they are the only middle tank inhabitants in a 5.5).
I will post about my tank and it’s current inhabitants in just a sec, but I wanted to comment on someone’s suggestion that you do a group of dwarf puffers. I had one for a while to deal with little plant eating snails that tagged along into my tank on a cabomba plant. He was SOOOOOooooo cute! They really are tiny and would not mind your little hex tank (as others have said you are even more limited than a normal 5.5 because your surface area is reduced due to the height to width ratio of those tanks.)
However, I would *not* recommend having more than one in a 5.5.... maybe two tops. They are VERY territorial, and most sites I have read recommend 1 per 5 gallons, again not because of waste (although they are extra big waste producers like goldfish) but due to their need for territory once they mature.
That said, however, being limited even to just one dwarf puffer would be well worth it because they are so entertaining and obviously very smart. If you have a lot of landscaping (ie plants, rocks, driftwood, ect) for them to explore they may even leave a few non-invasive other species alone (They have been known to tolerate ottos well, and mine never bothered the Cory cats either). .... *IF* and here is the BIG if... they have plenty of live snails to hunt.
That is what they do all the time... hunt... They go over every surface in the tank, staring down everthing, structures, plants, and inhabitants with their beady little eyes. Some people say they even start to stare at you, once they learn your face equals food! But you will have to get snails from the pet store or breed them, because the puffer will clear out the tank of every snail in about two weeks. You could defiantly have the dwarf and two ottos. If you find a less aggressive puffer as I did (and a few others I have talked to) you may even be able to have two Corys too. But if you have more than one puffer, it will get very aggressive and territorial, so in that case having any other species is out. Either way.. introduce them slowly, watch your levels, do frequent water changes, and put the puffer in last.
Hypatia
...who just noticed on preview that her signature line is already out of date. More on that in just a sec...