I think this should be fine, but as mentioned in the Comments you'll want to assess for yourself.
While the knockdown fasteners that Ikea use here are not noted for being a super-strong way to put things together they aren't weak either, and the bolts or cams would have to literally tear out from the chipboard/particleboard for this to fail.
In terms of resistance to racking (any leaning side to side or back to front) back-and-forth motion is a non-issue. Side-to-side motion is a potential for concern however, but the top links the two units quite securely; so IMO both would really have to fail in order for this to collapse sideways.
I just don't see that happening under a static load.
But you must plan for unexpected dynamic loads, which could include just someone hitting the edge of the desk with their hip as they walk past.
Note: if you decide to go ahead, do ensure that all fasteners are tight before you fill the tank with water. Don't over-tighten, but make sure nothing is loose before putting this under load.