setting up a salt water tank

Question
I now currently have a 20 gallon fresh water tank. In the future I would like to make this into a 20 gallon saltwater tank. Would it be ok to use the same filter? I think it would be ok because it is a power bio-wheel filter, plus my friend runs one of these in his saltwater tank. Also is a protein skimmer required for a 20 gallon saltwater tank? And besides ph, alkilinity, nitrite, nitrate, ammonia and temperature knowing this from the past with my freshwater tanks, what else is essential to know of for starting out with a saltwater tank? P.S. I will just be starting out with  a couple of fish and eventually some live rock. Thankyou, I know I'm asking to much!

Answer
You won't need a protein skimmer until the tank is up and running with livestock. Other than what you mentioned as far as what to be aware of is salinity. You will need a hydrometer to measure the salt content of the water. The start up fish should be a couple of damsels. They will help cycle the tank which takes about 4 weeks. The use of live sand and some live rock is recommended at start up. For a complete and detailed start up go to about.com. Click on hobbies and games. Next page click on saltwater aquariums. You will be at the site of Stan and Debbie Hauter. Take the saltwater 101 course and do some reading. There are plenty of articles to get you started. Gerry.