You already have neons and otos and bettas in your other tanks (all of which could have worked) so I assume you'd like something different for this one. The obvious question is what sort of fish do you actualy like? There are just so many options to choose from - even with a small tank like this.
If you'd like something a little more unusual, maybe an african butterfly fish is your thing, or the dwarf puffer mentioned earlier. Alternatively, research killies - you'll find there is a huge number of killifish species that would thrive in that size tank - it could even sustain several generations if set up carefuly. If the tank turns out to be 10 gallons, also look into shell-dwellers - n. multifasciatus is the smallest and would be a very interesting inhabitant. It would be a little more of a squeeze in an 8 gallon but may still be possible if you're careful and do your research thoroughly.
If you'd like livebearers (keep in mind that you'll either want to go all one sex or 2 females to every male and be prepaired to get rid of fry) there's quite a few options. Of the more common ones, platies (I'd go for 3 females), guppies (either go for 4-6 males or a trio - if you want females 3-4 would work instead as females are larger than males) and endler's (same as with guppies) would all be options. I'd probably go with endler's and get a group of 5 males.
Of the less common livebearers, heterandria formosa (NOT gambusia affinis) - the mosquito fish - would work very well and should not require a heater either. They only get to about three-quarters of an inch so they are tiny. You could fit 8 in an 8 gallon quite comfortably. I suggest using a sponge filter if you go with these and watch the sex ratios - 2 or more females to every male. You WILL get lots of fry if you mix the sexes (though, with fewer hiding places and if you start off with just one trio for example, it'll take a little longer for your tank to become over-stocked). There are some other interesting livebearers out there but, realy, they aren't worth it if you don't have the room to breed and raise fry (wild swordtails, goodeids, brachy etc - some of which would fit and some of which would not
).
If you're after a small schooling fish or mini-community, look into pygmy cories, spotted rasboras and myer's hatchetfish (the smallest hatchet). Keep in mind that all these fish school so make sure they are in a big enough group. You probably won't be able to combine more than 2 species in an8 gallon though.
Somethign else you may want to consider is getting a trio of cherry barbs (1 male, 2 females). I'd preffer at least a 10 gallon for these but a heavily planted (or just a tank with lots of ornaments) 8 gallon would work as well. They get to 2" so you wouldn't have room for anything else realy. You could try including a single american-flag fish though (flags are a cichlid-like, behaviour-wise that is, killi).
If you like bettas, you probably like gouramies. This is my personal favourite group of fish so it's the one I'd most reccomend
. Try to ignore my biased position and make sure you research everything yourself...
Firstly, of the more common types, honey gouramies (colisa chuna/sota) would work - either a single male, a trio (1 male, 2 females) or a group of (up to 4) females. Another quite common species is the sparkling gourami (trichopsis pumilus). AA trio would just about work. They stay very small and a few pygmy cories or an oto or 2 would also probably fit in.
Of the less common, licorice gourami species - and they're quite varied - would work as a pair. You do need to be careful with these though as they are rather fragile and should, idealy, be in a heavily planted tank. Small schooling fish or bottom-dwellers can work as tankmates but be especialy careful not to over-stock in case water quality suffers. There's also a huge number of wild betta species that could work if you take the time to look around for info.
If i had an 8 gallon to my disposal (and wanted something hardy), I'd go for a single male honey gourami, 4 pygmy cories and 4 male endler's. If I could get my hands on them and was prepaired to put a little more efffort into it and do some extra research, I'd be bringing a pair of licorice gouramies home to a very heavily planted tank containing a few cherry shrimp for tankmates.
Which reminds me, don't rule out dwarf frogs or snails as your tank's main inhabitants. HOWEVER, dwarf frogs need live worms to survive and apple snails (the most commonly sold) get to about the size of your fist and are ridiculously messy. If you are prepaired to dedicate your tank to either of these, fine, otherwise, don't keep the snails with fish in an 8 gallon as you'll over-stock and don't keep the frogs if you can't feed them (BTW - 2 gallons per frog so you could keep something like 3 pygmy cories and 2 frogs for example).
But most of all, don't rule out shrimp. You don't realise how entertaining these creatures can be when all you see is ghost shrimp hidden away in a tank dominated by big fish. Try one of the larger filter feeders or some of the colorful scavengers - the bright red-pink cherries, bright green ones, bumblebees etc There are a LOT. If you plant a tank heavily and add lots of colorful shrimp, you can get the same effect as when you add fish - only you can fit more shrimp in than fish and they'll even take care of algae and clean up rotting debris (though yu should feed them as well - ordinary fish flake/pellet foods seem to work fine). If you're lucky, they'll even breed for you.
Like I said at the start, choose something you like. Just make sure to either clone an existing tank or fishless cycle before adding any fish.
