Are u wanting reef tank with coral or fish and live rock? So you know what tank im guessing the 75 gal
, what else you will need: light (depends on if you plan on keeping coral or not, &if u want coral what kind softies/leathers or sps
) softies are good and are usually the easiest to care for some type of t5 bulb lighting and will be cheaper. To grow lps
anemones require very good lighting looking at mh
or leds(reef leds).
filters; you need different filters most people will use sumps/ refuigiums for saltwater aquariums. You can buy then very exspensive or you can make the diy sumps. Usually a good idea to use breeder tanks to do so. They dont have brackets and you will have more vertical room using them.
If its a fowlr
(fish only with live rock) then you can get away with really good hang on back filters but I dont reccomend it. The reason why you limit youself with them. With a sump you can an ato (auto top off) which can keep your water level exact bc of water evaporation, crucial in marine tanks bc sality can change and its a pain not to have one. If you decide to have a type of reef controller/ or apex you get probes to measure your readings. Would you want all that stuff in your display? I want the least amount of wires ans equipment in my display as I can have.
Next you will need a return pump usually 7-15 X volume of you tank is what you want. I like the quiet one pumps.
some things you will need heater, marine test kit I use api brand. Substrate and rock, dry rock is alot cheaper prolly around 60 to 75lb for a 75 gal
tank. Look at reefrock.com or marcorock.com. You will need to sit the in buckets for about 2 weeks to a month with heaters and anything to give the rocks water movement. This will help from having a bad diatom outbreak which many of us went through.
You may want to get some powerheads to keep the water moving around in your tank. If you have the money get magnetic powerheads the ones that have suction cups fall alot. Hydor, sicce, deceng prices ecotech I would love to have but cant drop 450 on powerheads just yet.
the list goes on to what you want tanks can get very complex you can get uva sterlizers, protein skimmers(not neccessary, but will benefit your tank greatly).
to get you started I would do a diy sump, get a return pump 7-15 X volume of your tank. heater, light (you dont need anything great to start off with, but until you atleast have t5's I wouldnt get any coral), sand, rock (dry or live) live rock is exspensive (get dry rock and a couple pieces of live to seed it with). I believe marcorocks you can get part dry/live packages.
Hope this helps