Reinforcing Shelf to Support 500+ Pounds (for Fish Tank)

I want to make my best effort to support a shelf in my family room so that it can hold a fish tank, total weight around 500 pounds. I have one of those wall nooks where someone once thought a TV would go, but is now largely useless. I want to put a fish tank there. The dimensions are 52” wide x 30.5” high x 26” depth. I removed the 1/4” plywood shelf to reveal 2×4 frame screwed (3” screws) to studs on three sides. The 2×4 frame does not have anything supporting it from underneath (in the pics below, that is some cement board about 7/8” down to insulate from the fireplace). There are 2×4 studs running on 3 sides, but obviously not the front side.

The fish tanks dimensions are 48” wide by 13” deep, so basically it will sit across the entirely of the width, but will only go half-way back. Unfortunately I want to place the fish tank towards the front.

MY QUESTION: What is the best way to shore this up so that it will support the 500 pounds of weight? I was planning on using Simpson Strong-Ties to strengthen the framing. I was also going to add some 2×4 cross sections. I would think the point of potential failure though is connecting the frame to the studs. I was planning on using 3/8” lag screws and washers (pre-drill the holes to avoid splitting) everywhere I can to better attach the framing to the existing studs.

Anything else I can do? Everything inside the nook will be obscured when I am done (plan on putting picture molding around the entire thing, with a board for access too). Thanks in advance for any help.