Changes will need to be made but there are many different ways to set up a marine tank.
Firstly, probably the single most important thing to be discovered in the marine keeping hobby is Live rock and its properties. This stuff alone has allowed us to keep things that were only considered the stuff of dreams (or deep pockets) a couple of decades ago.
If you get good quality live rock and lots of it then you simply wont need another filter its as simple as that.
Live rock fulfils 2 purposes, it looks great and its filters the tank at the same time.
Ok I admit its expensive stuff but when you look at the cost of this item over the long term then its really value for money.
1lbs per gallon of tanks is the recomended minimum.
Ok, live rock issue covered now we turn to the externals...
An external filter can operate as a filter but thisis usually because it has some ofrm of media within it for the bacteria to grow. THe downfall of these filters is tey can produce high nitrates as there is no way to turn Nitratea into harmless nitrogen. (Live rock does do this by the way).
I have used externals alongside liverock but the externals were only filled with sponges as this helped to polish the water. The rest of the cannister was kept empty as this is a good arrea for Pods (small crustaceans) and sea squirts to grow... this in turn is good for hte tank.)
Another bonus with a cannister filter is that you can place treatments in the box. Its easy to have Carbon, Rowaphos or the like in here whereas its far ore difficult with a live rock only system.
Skimmers:
Not needed but highly recomended for most systems that operate a liverock base for filtration. A good skimmer can remove much of the nutrients fromthew ater before it even enters the nitrogen cycle. There is an arguement that good skimmers can "Overskim" and remove more fromthe water than is needed. I personally prefer to not skim (due to the system i use) or "Underskim"
(I havent gone into mud or Deep sand bed filters as these are usually sump based filters which i assume you wont be using)
Lighting:
Any light can be used to ang over a marine tank as long as its a fish on ly system. (its not essential for live rock either). Fish dont mind what type of light they have but the same cannot be said for corals.
Depending on hte type of corals you might want ot keep will depend on the type of lighting you will require. You mention its a deep tank and if this is true then you will probably need halides for the tank if you keep corals. Halides have the punching powere to hit intense light into deeper tanks. They generate alot of heat but there is no question over the light that they can produce.
Compact tubes (T5s) are an alternative. They are usually cheaper and they are more powerful than normal fluro tubes (T8s). However, whether they have enough punch power to hit into a deep tank is debatable. I would guess that you will be fine keeping soft corals in a tank of this size with T5s. If you want something along the lines of Hard corals then Halides are probably a must.
Sand:
I recomend you use Aragonite sand as this is a great way of buffering the tanks PH reading. Other sand just dont seem ot be able to keep it stable IMO.
Lastly, as the tank is 2nd hand and has held freshwater tropicals, its important to know its history. (thankfully you will know this already). If the tank has ever had medication in it that contains copper then i would not use it for marines. Copper is deadly to marine life (Corals and inverts) even in the tiniest quantities. COpper also has the tendancy to get soaked up like a sponge to be leeched back out at a latter date. (even in cleaned tanks)
Hope this helps
