Just anchoring it in the drywall won't provide all that much protection if all of the weight is above the anchor point.
First, if it's against the wall and has something on each side (does it) I think you are overly worrying.
But people anchor stuff to drywall all the time. While yes, someone trying can break it loose, between the natural inertia of the tank and the strength of a drywall anchor I think you're talking a fully malicious act, not an accident, to tip it.
A couple of L-brackets sticking out an inch or so, a hole in the back (if there is a back) to reach to put a screw in, and some anchors and you're done. Something like this:
To pull those out you have to be really destructive.
Other alternative is to build a stand that's a bit like a pyramid, a bit wider at the bottom. This could be inconspicuous with some L brackets coming out from the very bottom to provide a wider base (go look at how server racks have permanent or slide out braces to keep them from tipping forward), or could be shaped that way gracefully. Maybe something just 30% wider at the bottom than the top makes for a great increase in stability.
But if it was me, I just don't think I would worry that much. Find out when the cleaning crew cleans, and have the lights on during to give it a bit more attention.
By the way, I'd worry more about the cleaning crew unplugging it and forgetting to plug it back in than knocking it over. Most places I've been they were notorious for that. Could be a problem on the weekend. Plug it into an inconvenient outlet!
PS. Our make a bookshelf, with it as just one part of the bookshelf, so you have some a wider and deeper base, then it also fits with the decor?