I have a bit of expertise in this area from doing semi-professional photography work for a few decades. People ask me this question on a frequent basis: "what kind of camera should I get?", which, in the DSLR world, is actually the wrong question. The correct one would be "what kind of lens should I get for this application?".
I say this because any, and I do mean any newish DSLR camera from Nikon, Canon or Sony will absolutely allow you to take awesome photos and videos of your tank. Or more accurately, I should say that the camera's will have no limitations preventing you from taking awesome photos and videos of your tank.
One aspect of this that may help you narrow your choices is that 1080p versus 4K video is largely irrelevant for this application. For sharing where folks are going to be viewing the video on a typical computer monitor or phone, the extra resolution is completely wasted. 4K really only matters when you're getting into 65" TV/monitor sizes.
From the standpoint of lenses, the most important aspect for your application is the close-focus limit. Many, if not most, DSLR camera lenses that come as a package deal with a camera body will be either barely suitable for your purpose, or unusable for your purpose because of the particular lenses' close focus distance limit. For most of these lenses sold as a kit, that limit is about 3 feet (but it varies greatly depending on the lens). That means that it will be absolutely fine for full-tank shots and video, but not so great for taking images of individual corals.
Nevertheless, if you don't currently own a DSLR, it may make sense to buy one of the packaged kits (DSLR, batteries, lens, sometimes a case), because the lens that comes with it will be useful for everyday photos as well as full tank shot photos. Once you get some experience with the camera/lens, you may wish to buy a separate "macro" lens. These lenses have a close-focal-distance limit measured in (a few) inches, and are ideal for photographing corals - especially the ever-popular individual polyp shots.