IF you want to pursue a small tank like that 18 gallon for a saltwater then I suggest you head to the website www.nano-reef.com
It is a website full of articles geared towards setting up a small reef tank like you have, including which fish and inverts you will be able to use. Also which corals, though I suspect you will need a lot more light thaan what you have for some of the hard corals.
Specifically it mentions methods of filtration. You need live sand and live rock to filter and you don't need the biowheel, which will eventually become clogged up and useless.
Check out the page. I have found tons of useful info there.