For a fish only tank - a term we refer to tanks that do not have coral in them, doesnt mean there is only fish, there can be shrimp etc - fluro lighting is fine.
If you have the skimmer, use it. If not - its not mandatory, but alot of people will recommend that you do use it.
If you dont, you need to keep an eye on waste levels.
Have you read up on the
Nitrogen cycle? This is the reason you need to wait at first before adding fish.
R/O units are used to filter tap water for topping up your tank.
Evaporation takes freshwater out of your tank, leaving behind the salt, so your tank slowly becomes too salty. If you have an LFS nearby that can sell you RO water, it would be a cheaper option to start with. An RO unit is usually cheaper to buy and use in the longrun, but you can always get one later.
You dont need a filter. All its good for is putting more liverock in or running empty for flow. A powerhead is a good idea.
Obviously you need a heater and a salt mix or access to natural salt water.
What kind of substrate are you planning on using? Sand is the cheaper alternative, otherwise
crushed coral is good, or marble chip, etc. Alot of folks like the barebottom look, but id recommend using something for your fish's sake.
The best thing to do is read read read! Use the search function and start looking!
