are you sure there's no fancy filtration or lighting I need for SW fish?
nope, pretty cheap for a 20g the only thing that stays expensive is the rock, live rock is around $6/lb if you have the patience, get all dry rock for around $2/lb and wait an extra few weeks for cycling.
for a 20g sw which i just upgraded from, its pretty cheap, and if your impatient like me, you can go both live rock a dn live sand but its more expensive, if you can wait about 6wks you can go all dry, but for the sake of it lets say...
40lbs of live sand (type is your preference) - $50
30lbs of rock - live (reef) $150 - dry (fish only) $50
good hob filter 30-40g rated (overkill is good) -$50
a pair of good powerheads (koralia nano 240s) - $75
a good hob skimmer (reef only) - $75
good led lighting - fish only $50 - reef (taotronics dimmable fixture) $150
so for a fish only your looking at around $300
and for a reef your looking at close to $600
initial cost is very high to most people especially coming from a freshwater background, but its cheap in the long run, with water changes and other minimal maintenance its very worth it, IMO, i went from 7 fw tanks to 3 sw tanks and now im working on a 4th, its a cake walk once you get in the groove of things.
hope this helps