A 20 Gallon Long is a great starter tank ... Started with a 29, it crakced after a few months ... Bought a 20 Long. The reason I like the 20 L or a 30 L is it gives the fish more room to swim, since most fish swim side to side and it is much easier to light, since the tank is not real deep (10 or 12 inches I think..)
Anyway .. The Ocellaris "Percs" Clownfish are very cool, and mine are doing very well in my 20. Just to give you an idea, here is what I have in my 20, and the cost that roughly goes into starting the tank:
-20 Gal Long $25
-25 lbs LR $ 100
-25 lbs LS $ 20
-Bag of Instant Ocean $ 15
-2 Filters $ 40 to 50
-3 Maxi Jet Powerheads $ 40
-130 Watt PC (Optional to get) Approx $ 100
That would get your tank set up fine. Its not cheaper getting a smaller tank by any means actually. The time and effort put into is alot greater then that of a larger tank, but it is an awesome little tank to have.
RB is right though, that bigger is better. I want another tank, and can afford it, but I just dont want one right now. If you think you are going to want to add a few smaller fish, go with bigger. If you just want a couple clowns and other inverts go with a 20 or 30 L. This hobby becomes addicting though!
Heres an idea to what you could put in the 20. These creatures are in my 20 and doing fine with weekly maintenance and daily feeding:
-2 Ocellaris Clownfish
-1 Red Sea Starfish
-4 Featherdusters aka Tube Worms
-10 Red Leg Hermits
-10 Turbo Algae Grazers + 1 Small Conch Hitchhiker
-3 Bumble Bee Snails
I will be adding corals soon, as well as possibly a small Goby, such as a Yellow Watchman and a few small Sexy Shrimp.
A small tank is very cool, but demanding as well. Things can go bad quick, fortunately nothing has for me yet. **Knock On wood**
If you think you are going to be in this hobby for a long time and have the money, get a larger tank. If you dont have the funds it is okay, and a smaller tank will do. But alot more maintanance and care will be needed for the tank.
Just dont overstock. That is the main thing. If you want more things. Go big. A few small things, go smaller. Hope this helps and good luck. This is a great hobby.
HTH