One of the first things you'll be told when you get a new aquarium is that it's very important to wait for the tank to "cycle" before you add any livestock. The "cycling" of a tank is more or less the period of time during which the bacterial and microbial populations of the tank proliferate to the extent that they can complete the major steps of the nitrogen cycle, converting toxic ammonia and nitrites into less toxic compounds. "Cycle" is written in quotations marks because there really isn't any reason to think of this phenomenon as a cycle. It's probably better thought of as a "breaking in" of the aquarium (though, certainly one can understand why "breaking" is probably not the best word to use when speaking of aquariums... even in abstract expression). However you want to refer to this first several weeks of your aquarium's life, it's important to monitor your ammonia, nitrite and nitrate levels during this time. Adding livestock before your tank has completed "cycling" will likely result in disaster.
So, how do you add livestock to your aquarium? In a word, the answer is; SLOWLY!
Step 1: Live Rock First
(first 3-5 weeks)
In order to build up bacteria populations, you'll need a source of ammonia (i.e. nitrogenous waste). There are several ways to get this (some more creative than others); but the best way is with live rock. Add all your live rock and sand BEFORE you add anything else. It will take at least 3-5 weeks for your live rock and sand to "cure" (i.e. for all the dead and dying matter to degrade away). You don't want any coral or fish in the tank while this is happening. Your ammonia levels will spike and probably get so high you'll be able to actually smell it. After the ammonia spikes, it will start to fall sharply, corresponding with an increase in nitrites levels. Following a spike of nitrite, you'll begin seeing a steady rise in nitrates. Once your ammonia, nitrite and nitrate are all at acceptable levels, this is when you can start to think about adding other livestock.
Note: Remember to lightly feed you tank (with whatever variety of marine aquarium and sea food you choose) even during this initial cycling period.
Using Damsels to "Cylce" a Tank
Unfortunately, many people still advice new marine aquarists to use damsels to start off a new aquarium. Even though most seasoned aquarium store employees and managers know better by now, for whatever reason, many still continue to suggest damsels as "beginner" marine fish. There are a couple reasons why using damsels to "cycle" a tank is almost always a bad idea; 1) damsels may start off small, but many get quite big, 2) they often grow up to be very aggressive, "mean" fish, 3) no livestock should be used to start off the "cycling" of an aquarium. Though it's true that damsels can usually tolerate higher levels of nitrites and other toxins, this is no reason to purposely expose them to a premature tank. The best way to start a tank off (to start the "cycling" process), is with live rock and/or live sand.
Step 2: Clean Up Crews, Detrivores and Algae Eaters
(after week 5, or whenever the tank is done "cycling")
During the first several months (and probably even up to a year) after your tank has "cycled," your tank will likely experience a whole series of algal, diatom and bacterial blooms. Snails, worms, and other detrivores and algae eaters can help keep some of these blooms under control. They won't keep your tank completely free of algae, but they will help. Start with ~one grazing snail for every 2-3 gallons of tank. Be cautious of online vendors selling "clean up crews", algae-eating or detrivore "packs" or "kits. Though well meaning, these kits and packs often include animals you don't want, such as crabs and sand-sifting stars.
Note: Be careful of hermit crabs. Though they can be good at "clean up" and algae grazing, the can (though not always) grow up to be a nuisance to some corals.
Step 3: Corals and Anemones
(weeks 8-9, or ~one month after initial cycling)
It is quite prudent and wise to wait at least another 3 to 4 weeks before adding corals and/or ornamental anemones. Not only will waiting give your tank more time to mature, it will give you more time to get the hang of things and to ensure that your tank parameters and chemistry are relatively stable (or at least predictable). Start with corals/anemones that are tolerant and easy to careful. However, do not select any coral that you do not always want to have. Some corals, like leather corals, that are tolerant and easy to care for also produce substances that can be toxin to stony corals. Therefore, it's important to plan in advance what corals you will want to ultimately have.
Although corals don't add the same bioload as fish do, more corals means more food. As you add more corals and/or anemones to your tank, you should keep in mind how much additional food you will have to add to the tank to feed them all. The more you need to feed your tank, the larger your biological filter capacity will need to be. You can build up your biological filter capacity by increasing feeding and coral additions slowly.
Step 4: Fish
(weeks 11-12, or ~one month after initial coral/anemone introduction)
If you waited this long to add any fish to your reef tank, congratulations! You are both wiser and far more patient than most new aquarists. You will not regret it. By this time you've likely thought long and hard about which fish you want to add to your tank. You've probably suffered through a few minor crises and know a lot more about reef keeping than you did 3 months ago. However, don't let yourself get impatient now. Add your fish slowly (no more than one fish per week). Also, remember not to overstock. If there is any true "rule" for how many fish you can safely add to a reef tank, it's the fewer fish the better.
