Large Object Between Mini Monitors on Stands...

I would imagine a horizontal large object like a drafting table would be less objectionable than a vertical large object like a deep cabinet between speakers.

Also the clarity of the image depends largely on the clarity of the water.

I heard with tropical fish between the speakers the image has a tendency to wander.

I've had a four to five foot long aquarium in between my speakers since I was fourteen years old (36 years ago) so I don't even know what nuthin' between the speaker sounds like. I do have exceptional imaging, though. If you like tropical fish that is.

...'Is this large object going in the same room where you are putting the speakers on the shelf ( because there is not enough room for speaker stands ) ?'...

It's already there. I'm figuring the front legs of the (steel) drafting table could be fitted with side shelves so as to function as quasi stands which would move something like a Dynaudio X12 or a ProAc Tablette approx 30" out from the front wall and 27"ish from the side walls with 4' of not necessarily empty space between inside edges of the speaker cabinets and put the tweeters at much closer to ear level for the critical listening position approx. 6' from the speakers.

In my experience large objects between the speakers tend to degrade imaging (particularly depth) and clarity a bit. Whether or not that's a deal-breaker is your call. I could live with it if the choice was that vs no music.

Is this large object going in the same room where you are putting the speakers on the shelf ( because there is not enough room for speaker stands ) ?

If so, dont see how this would be a problem since your speakers arent in the "normal" position.