I've now had my marine tank up and running for over six months. It's an 18 gallon tank with two fish (a clownfish and a blue devil) and a marine hermit crab. The animals, with the exception of a green chromis fish, who recently became ill, have all done well and are beautiful to watch. Sarah loves them, especially "Mr. Crab."
For those interested in starting a simple marine aquarium, here's what ours required. If you've had freshwater fish, you may be able to use some of your equipment.
An all-glass aquarium (ours was 18 gallons; the bigger the better) with appropriate stand
A hood with lighting source
A heater
A thermometer
Coral sand (We used "live" sand, and I think it made a world of difference.)
Decorations such as coral, shells, background, etc. You decide if you want fake marine-type plants.
A bio-wheel type filter
A test kit to test for nitrites, ammonia, pH, and nitrate
A hydrometer to measure the salinity of the water
An airpump with tubing and airstones
Salt-water (either pre-prepared from your local pet store OR made at home using packaged marine salt and bottled water.)
Books on marine aquariums
A water-vac siphon system or "Python" cleaner is GREAT and worth the money.
NOTE: I don't have a protein skimmer. I thought all salt-water tanks had to have a protein skimmer but was told otherwise at my local fish store. My aquarium has done well without this added expense.
FISH: Be sure to research your fish before purchase. Most damsels and clownfish ("Nemos") are good to begin with, but some can be aggressive. Remember to check their adult size! Some of these little jewels grow like a Great Dane puppy!
ESTIMATED COST: Probably between $200 and $500 for an 18-25 gallon tank. The lower end if you have some of the equipment or can get a used tank, the upper end if you want a bow-front tank with oak cabinet and all the trimmings. Look for "package" sets if you don't have any equipment at all. I had a tank and some equipment and spent about $200 on coral sand, salt, decorations, hydrometer, fish, etc.. Fish can run from $3 for the nice, hardy, colorful but sometimes aggressive damsels probably into the $1000s of dollars.
Cost to Maintain: Maybe $15-$20 a month, probably less. That includes purchased salt water (10 gallons at .80/gallon for 2 water changes per month), bio-wheel filter, a minimal amount of frozen brine shrimp and flake food and electricity. Occasionally you'll need to change the bulb in the hood.
Go on to the next page if you want an idea of set-up and maintenance.