If your stand is painted black, it would be easy to build a small black canopy for it. It's a 55g, so the would should be cost efficient, and paint is also pretty cheap.
A canopy is basically a wooden top... it basically consists of five pieces of wood and an optional back and two hinges. There are numerous ways you can build a canopy, so look for some designs on the internet and pick one that would be easy for you to assemble yourself.
Removing your glass tops will greatly help your pH problems - and by adding a canopy, you will ensure that your fish don't jump out as easily and that the light from your unit doesn't shine in your eyes while your looking at the tank.
HOWEVER, if your only goal is to increase gas exchange and increase your pH, I feel like you could go ahead and remove your glass tops and keep the light you have over it. If you are really concerned about your fish jumping, you can buy a piece of what they call egg crate - it's the grate type of thing that they use on flourescent light fixtures... and you can cut it to fit the top of your tank. It usually comes in white, but you can spray paint it with black krylon paint. That would be a simple $13 fix instead of building a full canopy for your tank.