How can I reinforce a piece of furniture to put an aquarium on it?
This sounds like a bad plan. 550 pounds (or more, likely) is just way too much weight to try to hack something together. Even if you put a support in the middle, then you need to make sure it's  perfectly balanced or you're going to crack the aquarium at the pivot point — the glass tank is going to be very unforgiving of an uneven surface.

Plus, think of the enormous problem if your tank does crack or fall over. 50 gallons of water could cause a huge amount of damage. Make sure your homeowner's policy includes flood coverage that would pay for this if you do try it!

Just get a stand that's build to hold an aquarium. It will probably be cheaper than buying a massive piece of tempered glass.

(Although I strongly discourage this idea, if you are committed I would suggest that you carefully test your contraption. Get a bunch of bricks, bags of gravel, etc. that weigh a little more than you're expecting. E.g. if you think the tank will end up being 600 lbs all told, gather up 800 lbs of materials and stack it in the tank. Leave it for a few days. Jump on the floor. Throw a pillow at it. Knock on the glass. Lean on the cabinet. If it's going to collapse, you want to find out before you have all the water and fish in there.)