Hi oscardude!...i made the big switch a couple of years ago and have never looked back.
I am currently running a 50gal marine reef system with l.p.s and s.p.s corals and a few soft corals too.I have 2 clarkii clowns in my set up along with a few other fish.ive had my clowns for over a year now and they are my favourite fish.
Now back to your question...you first need to decide what type of species you want,as there are a few out there..personally id go for skunk clowns if you want a nano tank(small set up 10-15gals) or percula common clowns/clarkii clowns for a larger set up.
Personally i would probably strip everything out of a freshwater set up and start from scratch.this is because your freshwater in the tank will contain a masive amount of phosphate and nitrate,which is a nightmare in a marine tank,because it fuels algae....big time!
This is why any serious marine keeper will use r.o water,to eliminate nitrate and phosphate,aswell as all other trace minrals in tapwater.which should be used from the ofset in any new marine set up.
As far as equipment goes...a skimmer is a must! some people will say you can run a marine system without one,which is true to some extent,but problems further down the line are inevetable.especially if you want to keep a anemone to host your clowns.
The next best piece of advice i can give you is to read a marine water chemistry book,or any newly published marine/reef book.Because...lets face it,when you get the water quality right,the rest is easy!
keep me posted on your plans,i may not have answered you question entirely because there is too much to write in one post.im interested in your tank size and further plans with your fishkeeping..
