Ok well anyways.. there's some pretty straight forward comments going around here, and some of which are not at all necessary depending on what kind of livestock you will have in the tank.
For one thing, lighting is definitely not a must. If you plan to just have fish and live rock for a good amount of time, then the light fixture you currently have will do just fine. It is only when you start to wander in to the realm of corals will you need a better lighting solution. I'm guessing your using the 75G tank, so with a tank that deep you are going to want to look for T5 High Output light fixtures. I have a 55G tank myself, and from what i have learned, 75G tanks and 55G tanks are relatively around the same depth. So whether you are going with your 55G or your 75G you are going to need strong lighting. Generally speaking, T5 HO
's are considered pretty cheap for the amount of light they produce and their effectiveness at keeping coral alive and healthy.(Assuming you have the proper light fixture over your tank.) Then, we have the metal halides which are boatloads of money... but you are getting what you pay for. Metal Halides produce great quantities of light and apparently, they give a wider spectrum of light which is better for the corals.. or so i've been told. Metal Halides are really expensive, so if you are on a budget they certainly wont be anywhere in that budget haha.
Just something to note for future reference.. Assuming your 75 or 55G tank is close to 20" deep or higher, not matter what light fixture you end up bying, you will need to find one with a generally high wattage. If you want to have a wide variety of coral you are able to keep alive in your tank, look for fixtures that have over 320Watts. I have the "Current USA, Nova Extreme Pro" which is a 6-bulb T5 HO
fixture with each bulb being 54Watts. Now this is over a 55G tank with a depth of 21".. and with this fixture i can keep alive just about any coral, including anemones which need very strong lighting to live comfortably. Just something to consider.
On another thing, protein skimmers are HIGHLY reccomended for any tanks over 40G so it would be in yours and your tanks best interest to buy a protein skimmer as well as not slack on the one that you buy. Your tanks have A LOT of water, and an efficient protein skimmer is a must if you want that water to be clean and your livestock happy and healthy. Some of the cheaper but well-made skimmers are "Reef Octopus" skimmers, CPR skimmers, and a couple other ones i forgot the names of. Whatever you do, stay away from the skimmers named "SEACLONE". These skimmers are horribly made, are cheap, and usually break in a short period of time and ends up causing you to spend more money on a better skimmer which should have been done in the first place and would have saved you money. It just is not worth it to get a crappy skimmer.. just because it's cheap. Stay away from the ones you see on ebay for 30 or 40 dollars, because they will wind up giving you more headaches then results. Definitely do not slack in the protein skimmer department.
Another thing... You said you used "natural gravel"? What is that? Is that like the gravel that is found in freshwater tanks? They look like the ugly vibrant colored gravel for fish tanks.. except without the vibrant color? Haha. First off, it is good that you put a decent layer of sand over it.. but what will happen if you decide to get inverts that burrow in the sand? They will start to dig through the sand, and all of that gravel will begin to get pushed to the surface and will accumulate and cause nitrate problems in your tank. Now, it really does not seem like a life or death situation at this point.. or even in the near future.. But if you plan to keep this tank long-term and you plan to have some interesting and diverse creatures in there.. you might want to consider removing the gravel later on and replacing it completely with sand. It's better for the tanks ecosystem as well as its inhabitants.
Also.. Don't feel pressured like you HAVE to buy an RO
unit. It is definitely not one of the most important components of a saltwater tank, and saltwater aquarists have survived for years without one. Is it a good investment? Yes. Will it make all the boo-boos go away? No. It will definitely help in the elimination of unwanted metals and other elements that may cause algae growth and other growths, but i know other people including myself that use RO
water and still have some issues regarding algae and other wierd looking things that grow all over the tank. In my opinion, getting an RO
unit is a sound idea and great investment, or even using RO
water from supermarkets or pet stores, BUT if you are on a budget with this tank it surely is not a must.
Good Luck and remember to come back with more questions.