The OP asked what the difference was, I was using typical examples of the differences....
Water type: Yes, there will be marine salt added to the water
Yea, just bung it in with the fish, that will be fine, how much? Dont worry about that.... Lets not get too picky, I think if a poster genuinely does not know the difference between marine and fresh water then the correct forum would be a good place to start
FO and FW is adding marine salt to an SG of somewhere aroun 1.018 and above. I wouldn't class that as "LOTS!", would you? And I wasn't detailing how to set up the tank and what each parameter should be, just pointing out what the real differences are, rather than the perceived ones thrown about by people.
Here are a couple of links with varying information....
10 steps for FO
The above site makes the classic mistake of assuming the oceans where the fish we keep almost never change. I recommend reading Scott Micahel's ecperiences of diving and how he noticed the marine environment around reefs was far from the non changing entity most aquarists assume it is.
I myself was snorkelling in Fiji recently and the lagoon I was in had huge variations in temperature as the sun warmed up the smaller body of water and then cooler water flowed in from the ocean at high tide. I witnessed heat shimmers due the large temperature difference. Did the fish avoid these areas of substantial parameter change? No. They swam straight theough them without noticing, and these were Butterflyfishes, genreally considered as somewhat delicat in the trade.
I am also unaware of a pH under 8 being deadly to fish, I know my FO marine tanks seldom read above 7.8, so I somehow doubt that claim. If you look at point 5 of the link it talks about Live Rock, There is no Live Rock in a Fish Only, if you use Live Rock you have a Fish Only Wioth Live Rock (FOWLR).
I have complained more than once that almost all the pins down the bottom of this forum only handle more reef based settings and that none point out just how easy a SW tank can be to set up. Some have suggested I should write a pin, but as you will see below, how to set up a SW tank is hardlt worthy of a pin.
And a couple of good books...
The Reef Aquarium, Volumes 1 and 2, Delbeek, Charles and Jules Sprung
The Conscientious Marine Aquarist: A Common sense Handbook for Successful Saltwater Hobbyists. , Fenner, Robert and Christopher Turk
The first book is a reef aquarium book, unlikely to be all that great for a FO set up. Bob Fenner's book is very good, but does someone really need to read books before setting up a simple tank? If someone wanted to set up a small FW tank for a few neons would we send them out to read books, or point out just how simple it is?
All too often people on this board people try and p[oint out the huge differences between FW and SW when in reality there is not that much until you start looking at keeping inverts. I have noticed the worst of these (and I am by no means pointing the finger at you here) tend to be from people who have only kept FW.
As an example of how easy it is, I shall do the entire set up of a FO tank below in easy step form:
1) Get tank, fill with substrate and dechlorinated tap water and heat to temperature;
2) Add filter and then add salt until SG is around 1.018 or above;
3) Toss in a frozen prawn, or add flakes, or add pure ammonia so that the filter media can start to get online;
4) Measure the ammonia and nitrite levels until they have peaked and fall back to 0;
5) Buy your fish and put it in the tank.
That is it.
Take out the point about salt at point 2 and you have a simple FW set up. That is how little the difference is between the two.