It should have a weight tolerance listed somewhere in the assembly manual, or in whatever paperwork you have.
It is really depending on those 4 very small legs to hold the weight of the entire chest plus tank plus contents of drawers. It may be fine but it was not designed to support weight like you plan to put on it. It would be better if the tank was almost exactly the same size as the chest, so the perimeter pieces, or the vertically placed lumber, was supporting the weight. What would be even better than that would be a piece of furniture that had no legs, but rested entirely upon the floor.
If you can calculate the weight of the tank/water/rocks/whatever and get enough people to equal that weight, and all of you sit on the chest and kind of wiggle around to see if it moves, then you might have a better idea of how it might hold the tank.
I recently helped my neighbor one night after a desk he had a 29-gal
sitting on broke (had small legs holding it up) and his tank smashed on the floor - we spend the evening picking up fish and fry and pieces of glass, and vacuuming up the water and gravel. What a mess. I'm a little leery, so I'll play devil's advocate and rain on your parade - sorry!