Now i have a sturdy structure, but i need some surface for my tanks to rest on.

For that purpose, i routed a 18mm deep cutout on all the internal edges of my beams :

It's 18mm deep because 18mm hardwood plywood is common here.

So i routed the beams, and the foot assembly on the inside, until i could fit two 1030*520mm planks, one on each side of the central divider.

It was quite some work (hard wood is simply HARD!), and i had to make quite a few passes, i probably took 1 to 2 hours to route it to the correct depth.

One of my plywood sheet was too long because of reasons so i recut it on my table saw.

I had some chiseling to do to get the corners square (so that i can change the plywood sheets if i want to without any extra work : preferable than sanding the corners round each time!), and at last the plywood sheets did fit :

To remove it, i simply have to push from below. They can't fall in any way, each side is supported by a 40mm thick lip, 16mm wide, of hardwood.
This is a snug fit, so no moving around.
I know that the plywood doesn't have a great visual, but it's a very flat and solid surface, and it will be covered up with aquariums and some foam.

Anyway, i have the first level of my stand. Below that i'll store some stuff, along with water reserves, and later on, i'll build a second level that rests on this one.
For now, the next step is to make the assembly sturdier to lateral forces.
At this step, i can easily stand in the midle part between two sets of legs, and the wood doesn't flex.
I'm 90kg, so i'm confident in the weight resistance of the beams
(aquariums may be heavier, but weight will be evenly distributed, whereas i was standing in the worst possible spot, and tried to make it flex but putting all my weight in one spot.