I've the 3x4 Kallax. It's bracketed to the wall but still has slight movement. Definitely add additional brackets. Put tank in the centre. The top is a decent solid piece - though if you lay the unit sideways to have 4 across and 3 up then it's more stable & at a better viewing height.
I found this post by searching for 'Kallax' as I'm thining about putting a tank on one.
We have a 3x4 cube ikea Kallax and I'm going to put a 15g tank on top. But I started donig the math and realised the tank will weight ~75kg when full. I will use some brackets to secure the Kallax and prevent side to side movement.
Anyone have other tanks on ikea furniture? Am I going to end up with water on the floor and a smashed tank??