Can saltwater fish live in a 20 gallon tank?

It is possible, but there are often problems maintaining a stable environment (temperature, water quality) in a tank that size, and you'll need to be careful when choosing fish for a tank that size. My smallest (currently) is 10 gallons, but I've kept as small as 2.5 gallons.

My advice would to be to get a good saltwater aquarium book and do a little research on the equipment , fish, and invertebrates that could live in a tank of that size. Fish, snails, shrimp, and some crabs, plus a few low-light mushroom polyps would work in a 20 gallon, but I wouldn't try for any corals or anemones unless you would plan to upgrade your lighting.

Here are some saltwater links that can help you with equipment, set-up, and some of the livestock that are avilable. Look at sections labelled "nano reef" in particular:

http://www.apapets.com/MarineEcosystem/whatto.htm

http://www.peteducation.com/index.cfm?cls=16

http://www.tropicalfishoutlet.com/

http://www.fishlore.com/SaltwaterBeginners.htm

Good luck with the tank.

Do you have any theory what so ever what this is going to fee to have a saltwater tank? particularly there is not any longer something you could put in 10 gallon tank in saltwater. you rather ought to understand what you're doing. The 20 gallon is slightly greater beneficial, yet you won't have the capacity to over load it. Saltwater is very some artwork. Saltwater tank isn't something you could basically throw mutually. i understand I even have one-seventy 5 gallon. Did I point out the money. LOL

you might be able to have a few seahorses or small saltwater fish in 20 gallons but not many. The problem with saltwater is that it will start to wear away at your nice tank, lights are usually that first thing to go. If you want colour get small tropical fish like tertas (school fish) with a few medium ones (like guppies, platties, mollies or barbs)

Yes

But the more water you have for saltwater, The easier it is to keep it stable.

A lot fo people keep very nice "Nano" tanke from 12gal to 30gal.

http://www.nano-reef.com/

I just set up a very nice 30gal. Seahorse tank for my wife and it is the smallest saltwater tank that I have ever had.

Yes, but saltwater aquariums are much more expensive to set up than regular freshwater. If you are looking for color in your fish you could go with Guppies, Platys, and Mollies. Plus you can breed each one to make up your own colors.. Also Cichlids can come brightly colored. Good luck! :)

Yes, it's possible, but you have to buy extra aquarium salt from your local pet store, which can cost up to $20.