That is a standard top for an acrylic tank.
Why? Is this a new or used tank? Deflection is normal. The taller the display, the thicker the material must be. Bulging happens on acrylic tanks, especially the dead center point is that is the area that has the most area to bow.
Tank has been drained!
There is a piece of 3/8" acrylic on the top of the tank that has 2 main access holes and 1 for the overflow so there is a 3" border around the edges an in the center there is a 10" that transfers to 13" center "brace". I bought the tank used so getting ahold of the MFG might be difficult
Some type of frame around it or Euro bracing. In some of the pics it looks like it has Euro bracing. Either type is generally used to prevent bowing. 3/8" seems excessive. Have you contacted the Mfg about it? I would if you haven't.
If i put the level in the center and get the buble between the lines it mesures 3/8" Im not sure what you mean by bracing.
I'm no acrylic expert but I believe some bow is expected. That looks like a large amount though. How far out is it? Any bracing?
I personally would skip the closed loop idea as the addded hardware including the pump will still put you back a good amount of money. Your best bet if you are looking to save money is to use koralia's to generate a varied flow throughout the tank. The MP10 and MP20's from Ecotech are amazing but the MP10 is way too small for your application. You are going to need an MP20, and even better is to have two but at $450.00 a piece that is a good chunk of money to spend.
With a 75g tank I think the Mag 7 will be fine. Had one on a 65g tank for a few years without a problem. As far as the closed loop, I can see the advantage of not having to look at powerheads all over the place, but extra holes in the tank scare me. If your gonna go power heads I like the vortec's A LOT! But I would go with a MP20w instead of the 10w, that way if in the future you want to upgrade you can without the added cost of having to buy more powerheads. Some K-4's would get you by until you have enough for the Vortecs.
I look forward to seeing your tank progress! Your comment about melevs reef RO/DI going on sale made me laugh!