Mishelly00,
You need to remove the CC from your tank before adding the sand. Having CC defeats the purpose of having sand in the first place. Even if you add the sand on top of the CC, eventially it will make its way back to the top, and then you will have problems with stuff getting down into the CC and causing water problems. So first take out all the CC, then what I did was using a small bucket that is reserved for fish duty, I filled it up about 2/3 with sand, then lowered it in to the tank and slowly poured it out and spread it around. No matter how careful you are, it is going to cloud your tank for a day or so. As far as how much you need depends on what you are trying to do. If you want the sand just for looks, I would say 50 pounds would be plenty. If you want a deep sand bed (DSB) for de-nitrafication and for borrowing fish, you want to have 4-6 inches. I used 100 pounds in my 65 gallon which is the same as your 75, only it is 3 feet long and your tank is 4 feet long. So I would guess 125 pounds, but it wouldn't hurt to go as much as 150 pounds. Hope that helps some.