Hmm, ok... lol. Lots of questions!
Clownfish are very hardy. Try to buy two of them at the same time if you can. There are tons of fish choices out there, but with such a small tank, you need to stick with a variety of small fish. A pistol shrimp/goby pair are really fun to watch, do a little research on them. Clownfish are great, neon gobies are fun to watch clean your other fish, bicolor blennies are neat too. Firefish gobies are really fun to watch and one of my personal favorites. Keep in mind compatability issues, the amount of fish, and the aggressiveness of each one before your first purchase.
You got a long way to go before you can add any fish!
First and foremost, you can ONLY used purified fresh water. RO (Reverse Osmosis) DI (DeIonized) water is the best. Distilled water is also good, but really expensive. Use only purified fresh water for top offs. When your saltwater aquarium water evaporates, it leaves the salt behind, therefore you top off with fresh water. You need to go to the local water store, grocery store, or Wal-mart even and buy 20g of purified freshwater. Put it in your tank first, before anything else, and then add your salt. Instant Ocean is a popular favorite salt to use. In any case, measure out your salt and follow the directions to the letter. If you don't already have one, buy a Hydrometer. They will cost around 8 dollars, but you will be able to test the amount of salt in your water (salinity). You have to know your salinity level. Too low or too high and it won't support life. A salinity of 1.022 to 1.024 for fish only tanks is desirable. You mix your salt into your water with your powerheads. You will have to wait a full 24 hours before the next step.
The next step - look at your aquarium and see if the salt has desolved. The water should be completely clear and not cloudy. Measure the salt content with the hydrometer to make sure it is within the proper range. Install your filtration equipment - HOB filters, skimmers, etc. and mount your heater as well. Plug everything in and get it going. The water level in your tank will drop! Don't worry, that is a good thing. Next, add a few large chunks of base rock to the bottom of the tank. This will be your foundation for your other live rock. If you get any fish that like to dig holes, they won't be able to dig your foundation loose. Then add your sand to your tank. This will increase the water level in your tank. Then proceed on to the next step
Step 3.... Wait until your aquariums water clears up. Depending on the filtration equipment you are using, it will take anywhere from 24 to 48 hours for it to completely settle. Once your tank is clear, you can buy live rock and add it to your system. I like a lot of live rock in my tanks, because it helps with the biological filtration of the system. I put about 30 pounds in my 20g display and about 10 pounds in my sump. Live rock has bacteria, microbes, algaes, tiny crustaceans, worms, forams, diatoms, sponges, cnidarians and all manner of sea life growing on it. That is what makes live rock "live." Since the rock has been taken out of the ocean, and packed slightly wet, some organisms on your rock will be dead. This dead organic material has to decay, creating ammonia. Add your live rock to your tank and take out a little saltwater at a time in a clean container. Get your rock the way you want it to look. Caves, cliffs, overhangs, etc. Whatever you want. Once you have live rock in your tank, cut a small piece of shrimp and add it to the tank. Then put your lights on the aquarium and on an 8 hour timer. Then proceed to step 4...
Step 4.... Your is going to undergo a cycle. The Nitrogen Cycle to be exact. It's the process by which ammonia (created from the dead matter, and the decaying shrimp) will be converted into nitrite and then into nitrate by various bacteria that is building up in your system. Now is the best time to buy a test kit for ammonia, nitrite, nitrate and pH. Test your water every 5 days for ammonia, nitrite and nitrate. You will see a trend happening. Your ammonia will "spike" really high, and then it will level off and disappear. Nitrite will form shortly after, and then eventually ammonia and nitrite will completely disappear. Nitrate is then the end result of your cycle. During this time, your aquarium will undergo various changes. The first change you will notice is a brown algae bloom. This is diatoms. Diatoms are formed by excess silicate in your sand and water column. Eventually they will go away entirely. Next will be your algae stage, closer to when your cycle is complete. Algae live off of excess dissolved organic nutrients like nitrate and phosphate. Your tank will get an algae bloom, but not to worry, it can be maintained and controlled. Proceed to step 5...
Step 5... Once ammonia and nitrite are zero and your tank has set for a few extra days, add ONE fish. A hardy fish, preferably. I like clownfish because they are hardy. Some people prefer using a cheaper fish like a damsel as their first fish, but they are overly aggressive and they are little buggers to get out of an aquarium. Clownfish may be more expensive, but worth it. Don't add more than one fish per two weeks until you get the hang of it, and you have success. More aggressive fish are added last so that they can establish territories last. When you add your first fish, add a little macroalgae with it. Macroalgae is more than just decorative, it cleans the water, helps balance out pH, adds oxygen, removes heavy metals, and removes nitrate and phosphate from the water column. It also provides places for seahorses to hitch. lol
Step 6-20 and beyond... Read, read, read, read, read, read, read, research, research, research, research, research, and read read read some more. This hobby is very fun, and very entertaining. You just have to kind of get in to it and dig deep when it comes to the methods you want to use to filter your water. Your tank can be as simple or as complex as you want it to be. You, the aquarist, have to decide what is best for the health of your livestock.
I wrote this up for you specifically. This deals with an aquarium without a sump or closed loop.
Glad I could help.
