Need advice on 76 gallon Oceanic half circle aquarium

[Mcass]

I was recently given an Oceanic 76 Gallon Half Circle Tank. I know very little about it but would love to put it to use. From everything I have read so far, it seems that this could be a good opportunity to try my first salt water tank (I currently have two aquariums, one 54g for African Cichlids and another 30g for Goldfish). My problem, not knowing anything about the tank, is that it seems something is missing. In pictures I have found on the internet it appears that there is supposed to be some big black piece of plastic rinnung top to bottom in the back of the tank that covers a couple of holes in the bottom glass. I can also see, where it is missing, what looks like residual silicone where the edges of this piece used to be. What is this piece? What are the holes for? Where can I get this piece? How would I put it back in?

After that, what other equipment (specific suggestions would be nice) would I need to get to build a salt water tank?

Any advice you could give would be greatly appreciated.