Just my opinion, and I shoot neither aquariums nor macros that often, but probably just about any four-thirds or APS-C system camera (dSLR or mirrorless) with a good macro lens is what you're after. You can do this with a small-sensored compact, but you'll have a narrower window of opportunity and you'll almost definitely be forced to use off-camera lighting, given the limitations on high ISO settings and wider apertures, and the facts that most aquariums are shot in relatively low light, fish move fast, and it's all about controlling reflections and having very limited camera angels to work with.
You probably want to avoid full frame because of a) the added cost and b) the greater difficulty with getting a deeper DoF when macro shooting.
Expect to budget at US$1000+, though. You can probably get a decent macro lens for around $400, a body can be found in the $300-$500 range if you're willing to go used and aren't too picky about new features. And then you'll probably want to add on a tripod, a cable release or remote, and possibly some lighting gear and additional macro add-ons (close-up filters, extension tubes, etc). So, if you really want the image quality upgrade, it's not going to come cheap.
You can certainly hold off for a while and try to rock the X100S you've got, at least for shots of the whole tank, rather than the tiny denizens thereof. The X100 series more or less sees in the dark
. And you will definitely want to master using manual exposure, white balance settings (or shooting RAW and post-processing), as well as possibly using off-camera lighting (or at least finding a way to block reflections from the glass and to shoot perfectly perpendicular to it), and the X100S is certainly capable of all those things, and as a bonus can sync with flash up to 1/1000s (most dSLRs can only sync up to 1/200s, unless using high-speed sync).