It doesn't have to be expensive for a little FO
system if you do regular water changes, read up and you should have no problems keeping a few clowns in a 15; see http://www.bestfish.com/ssss.html
You will basically need:
- sand or cc
- good hob
filter, go for one that is rated for 30-40g fw
tank, like an emporer biowheel.
- salt mix
- tap water conditioner (you may already have)
You could buy some dry base rock for around $1 lb, check all your lfs
for dry base rock or "coral rock". Use that instead of fake decorations, it will turn to lr
in a few months.