Hey, Im a newbie too so take what I have to say with a grain of salt.
1. 20 gallons is pretty darn small for a saltwater tank. Keeping the water balanced and the fish healthy will be a feat, however, you will save money when it comes to live rock, substrate, and especially lighting.
2. You will need a filter, I would suggest one that is rated for a tank at least twice the size of yours. ( I personally like canaster filters, they have served me well in the past.)
3. You will want some agragaite sand for your tank. You may be able to find some cheap playsand (check out the playsand season thread) but I ended up having to purchase mine at the LFS
4. You should have high water movement, (think at least twice as high as your cichlid tank) With a 20 gallon tank You wont need a powerhead as long as you use a large enough filtration system. Just place the intake and output tubes far apart.
5. You will absolutely need a protien skimmer. This is something I have never used on my freshwater tanks, but is essential to keeping the water clean and pure in a salt water tank.
6. And most Importantly, expect to spend about 2 times more than originally anticipated. It is very expensive to set a marine tank up. I have a 37 gallon tank and I am very near the 1 grand mark and I havent put in a single fish yet!
7. Start with only fish and live rocks, but plan for a reef setup later, meaning that with every item you buy, make sure it will work well both with a FOWLR
setup and a reef setup.
again, I am also brand new, so take my advice lightly, but I am sure that some of the more experenced guys will agree with alot of what I have said.
Hardware: 37 gallon SW
tank and stand by Oceanic Systems, Magnum 350 Pro Canister Filter (half carbon), CPR Bak Pak 2R skimmer, Maxijet 1200 Powerhead, RenaCal 150 W heater, Compact fluorescent lights with moonlights. 40 lbs of liverock, 3" sand bed.
Software:2 Tank-Raised Ocellaris Clowns, 1 Purple Firefish, 1 Electric Orange hermit crab, 18 Blue Legged hermit crabs, 8 or so Nassarius snails, Xenia, Blue, green, and Green Striped Mushrooms.
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Central Illinois Marane Aquarist (CIMA) member.