There is no such thing as a "small" yellow tang. They are large fish and need larger tanks, around 75+. Anything smaller and they WILL outgrow the tank. Flame Angels also need larger tanks.
Now, being a newbie SW fishkeeper, I'll give you what you should know about SW fish care and tank set up:
Cycling: Cycling a tank is a must. SW tanks take about 1 month to 3 months before they are fully cycled. The cycling process is where beneficial bacteria (BB) grows on the live rock (LR) and takes out dangerous chemicals and substances in the water, filtering it. The first substance removed is Ammonia. Where does ammonia come from? Well, it comes from anything that decays. Dead fish, excess fish food, fish poop, fish urine, any form of animal waste has Ammonia in it. Ammonia can be deadly even to humans in high enough concentrations, for fish anything more then 0ppm (parts per milliliter) can and will harm the fish. Ammonia is eaten by a type of bacteria which converts it into an equally harmful chemical called NitrIte. NitrIte is equally harmful to fish, and should never get above 0ppm when fish are in the tank. Once nitrIte begins to form, more bacteria grow and convert it into NitrAte, which is only dangerous in high amounts. It should stay under 20ppm in a cycled fish tank. The entire process can be started in various methods. The one I use is to buy LR and put it in the tank, then test. If the LR is fully cured (another word for cycled) the levels should rise considerably from the die off of little creatures that live in the LR (they don't like air). That is good, it starts the cycle. If the levels don't spike, you can go to your local grocery store and simply buy some raw (or cooked) shrimp. Put one or two in the tank and they will decay, causing Ammonia to develop and starting the cycle. You can also go with the fish-cycle, where you add a hardy and cheap fish to the tank and it creates the ammonia needed to cycle the tank. This method is harmful to the fish used, and is not recommended by this fishkeeper. You will want a Marine Test Kit to measure ammonia, nitrAte, and NitrIte, as well as Ph and other levels. Ph is only important in that it needs to stay stable, and rarely changes.
However, before even getting the LR for the tank you will want supplies. They can vary in what you want depending on what you plan to keep. If you want a reef tank with corals, you will NEED T5 HO lighting, which is NOT cheap. With corals you need a Reef Test Kit, the Marine Test Kit doesn't cut it. However, fish only tanks are easier. You will need 2-3 powerheads. If the 20 is long, maybe 4. Any light will due for a non-reef tank, fish don't really care about lighting. The LR is your main filter, but if you want to have a HOB (hang on back) (which is what you have) it won't hurt to have one. HOBs are good for keeping water clear and house BB in decent numbers. However, with a HOB you need to change the media (I use pillow fluff from my local crafts store, non-cotton/feathers. Nylon I think) every so often to prevent clogging and simply to keep it clean. Always keep some filter media to seed the new stuff with. A skimmer is optional, I don't have one on my 30tall (yet). However, getting one is a good idea, as they do help with the tank look and stability. You will also need a heater, most SW fish are tropical. Besides powerheads and a heater you don't actually NEED anything, the LR does the filtering and a skimmer is optional.
If you want a sump for your tank, you will need pumps and tubing. A sump can hold extra water as well as unsightly (ugly) equipment like skimmers and filters. It can also hold a refugerum, which is a great place for non-tank critters to live (stuff that will become or is a problem in the main tank). Sumps are optional, and for a 20gal I would say you don't really want one. If you go bigger, like 30gal or more, then it would be an option. I have a 30tall that will have a 20 long for sump.
LR counts as a living creature, it can carry diseases, parasites, and other unwanted pest critters into a tank. Even after the cycle, wait at least a month before adding your first fish, then another after that before your second. This will ensure that there are no parasites or fish-hunting creatures that survived the cycle and came in with your live rock. Aipiatsa is a very common pest anemone for aquariums, a peppermint shrimp will have a 50/50 chance of eating them. If you see one, use Joe's Juice to nuke it, just douse the mouth (inside the tentacles) with it, the nem should die.
Stocking: For a 20 gallon tank several things factor into stocking: size, footprint (the surface area of the waters surface), and the amount of filtration. A well filtered 20 long can hold more then a well filtered 20 tall, or a low filtered 20 long. If your tank is a 20 long you can hold more fish and somewhat more active fish. However, a tang is never an option for such a tiny tank. The fish from >
this site< would all be options, but not all of them at once
Personally, I would recommend the following set up:
30lbs of Live Rock. 1 Merineland Penguin 350 (contrary to popular belief, you don't need special filters for SW tanks, any will do, unless it says "not for SW tanks"). A heater is needed, so I would have 1 of those rated for 30gal tanks. I would locate 2 powerheads on the back left corner and 2 powerheads on the front right corner, or one in each corner, to create a circular flow. SW needs a decent flow or else you will get dead patches of rock and water, which is bad for SW.
You will want a clean up crew (CUC) of 2 blue leg hermits and some nerite snails. You can also do cerith snails (I have those). Sponges and starfish will possibly come in with your live rock, if so they will also help keep the tank clean. As will feather dusters, but they are more rare.
Stocking for that tank would be as follows: 1
Ocellaris Clownfish, 1
Firefish, 1
Citrinis Clown Goby, and 1
peppermint shrimp. No corals and no anemones, both need high light, high maintenance, mature aquariums. Do not add all the stock simultaneously. Add the Firefish first, then the Goby second, the clown third, and the shrimp last. That tank will do very well as a low maintenance tank with nice, hardy livestock.
Do not get clams, scallops, corals, starfish, anemones, or sponges for the tank. They will not survive in it without special extras that you probably won't have. Hitchhiking nems/sponges/stars/ext do not count.
Money: Money will be the deciding factor in everything you do in Salt Water. SW is far more expensive to set up then freshwater. 30lbs of LR can cost over $100, the filters average at $60, the skimmers over $100 and lighting rarely under $100. My 24" Dual Bulb T5 HO light cost me $105, including the bulbs. It's awesome though. Powerheads are generally between $20 and $80, mine were about $30 each. Salt itself is rather expensive, my 25 gallons worth of salt cost $28 bucks (or maybe a little less/more). So save, save, and save some more. Don't go cheap on the initial investment either, I did and I hate myself for it. I've spend $400 more now on upgrading my tank to 30 gallons and reef level lighting. ($200 for LR alone). So save as much money as you can in the beginning before you buy anything. The fish I selected are inexpensive, but still not as cheap as FW fish. Clowns are $20, Firefish are $20, Gobys are $10 (at my store, prices may vary). So SW is not a cheap hobby. I'd say I've dropped at least 1 grand on my tank and it doesn't even have corals yet!
Welcome to the interesting and wonderful world of Salt Water! Where the tanks are bigger, the fish are cooler, and the rock itself breaths!
