theres alot of choice for a tank of that size, but depends what you want really, you wont be able to have any large cichlids cause the tank is just too small. is this your first time startiing to keep fish ? cause then i would reccomend just some hardy fish like livebearers, barbs. then you can get a hang of how everything works in a fish tank. then you can move on and maybe have a pair of convict cichlids or a pair of firemouths, these cichlids dont get that big and should be fine in a 30 gal, but then the pair would be the only fish you can put in there as agression starts when breeding and also because of the tank size would effect it, if you ever wanted to add more fish.
good luck with the new tank, any more questions just ask away
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