Thought I would chime in with my 1st post on this topic as it seems the answers are a little negative, but very understandable. Many people get off on the wrong foot with fishkeeping.

I've been keeping tanks for about 8 years, although currently I am without one as I have just moved. I've setup and taken care of dozens of tanks from as small as 1/2 gallon glass bowls to 200 gallon coral reef setups. I'm no expert, just an enthusiast, but I have read a LOT of material on fish keeping. I've also helped to train dolphins in an OCEAN environment, so believe i have a unique perspective...

They can cost a lot of money, especially saltwater reef tanks, and yes, can be a very time consuming hobby. But I truly believe that a properly setup reef/fish tank is one of the most amazing things behold.
I mean, c'mon, you got a freaking living coral reef in your living room!
And the price range of a tank is huge...the more money you spend, the easier it will be, but Aquarium keeping is HUGE on DIY projects, so I have seen 50 gallon setups put together for less than a couple of hundred bucks, and this includes lights and fish. But if you got money, so much the better in this hobby...

Some of the information you are getting from these posts are from people who it seems do not have much experience keeping saltwater animals. Animals such as eels and starfish and algae eater blennies do NOT belong together and certainly not in a 20 gallon tank!
I'm so sorry, luvfriday and silkyblue, for your experiences with horrible fish store owners. I have been victimized countless times.
Unless you know someone who is very good at fishkeeping, I generally suggest doing your own research before speaking with the fish store guy, and don't really "take his word for it" unless you have a good relationship with him. Online buying is awesome now. I suggest perusing some reputable online stores.

Make no mistake! This is a science! And a very new one, just like hydro. Read your ass off before you even consider buying anything larger than 30 gallons. In fact, read till your eyes drop ESPECIALLY if you buy a tank under 30 gallons, as this small volume will be even LESS forgiving (this is why the eel won't do well in 20 gallons!).
The key to success? GO SLOW. I'll tell you to go slow, but you won't do it anyway (none of us do! lol!). The SLOWER you go, the LESS expensive it will be, the LESS things will die.
PLAN everything like its your 1st grow :jointsmile:

I recommend to EVERYONE starting or experienced to read "The Conscientious Marine Aquarist" by Robert Fenner. Read this book and you are just going to be smarter. About fish anyway.

Sorry bout the long post, I just love saltwater fishkeeping. Done properly, I almost guarantee you will be HOOKED.
Let me know if you need any other resources or advice!