So, here we are. You have made your dream a reality. The conceived this tank in your head, planned how you were going to do it, purchased the equipment, set it all up and now your tank is cycled. YOU ARE NOW READY FOR FISH! I know it was a long journey, but your patience has paid off.
Now the choice is yours, well yours and good old mother natures. You may pick out everything you want, but nature will tell you whether or not they are compatible. Better yet, check their compatibility with the database here at FishDB and create a Virtual Tank to test your ideas. That way you won't go through the devastating mistake of purchasing a $50 dollar snack for one of your tank inhabitants.
Now that you are at this point, I have to bring out a few more concepts. First of all, every time you add a new fish to your tank, the system will go through a "mini-cycle". You may be asking yourself, "Why? Didn't I just go through this?" That is a logical question with a fairly simple answer. We have a certain amount of bacteria within or aquarium, it is capable of handling a certain quantity of waste within the required time to keep the environment "stable". So, every time you add another fish you are essentially adding more waste production via the fish and more food you may add. So, the bacteria would not be established to handle that next level of waste reduction without first "growing" to a level to handle it. However, this does not take very long at all and will be accomplished quite fast. The reason that this is so important is you do not want to shock your system by adding a lot of fish at one time. One or two will be fine at a time. The addition of three or four small fish can be handled by a large system with no effect, but do not add any more for a couple of days following. Allow your system to catch up each time you make an addition.
The second point I should make relates to the first. If you cannot add all the fish at once, what do you add first? Again, this is up to you, but here are a few guidelines. The more peaceful and shy a fish is, the sooner it should be added to a tank. This allows it to get comfortable before others begin to stake out territories. For example, burrowing fish will need to feel comfortable and calm to create a burrow. So, if you want one, it may be best to add it as one of the first. Obviously, this would mean that the more aggressive fish should be added last. The ocean, more specifically the reef, is all about territories. This is true with most creatures in nature. So, choosing the right tank-mates and adding them at the proper times can make a world of difference in the happiness of your tank. Then there are fish such as Mandarins and Dragonets, which need a well-established aquarium to thrive. So, you would wait 3-6 months for those. Again, you must research and research again.
The third point is when adding fish, invertebrates or corals to your tank you need to acclimate them. This means, slowly adjusting them to the conditions of your tank. You must realize that the water in the tank at the store may vary in many ways to your own, such as temperature, salinity, pH, etc. Not to mention when you place a scared fish in a small bag of water, there can be a lot of waste and temperature variations from the store to the point when you finally get it home to your tank. This is important because you can severely shock or even kill your new friend by just simply "dumping" it into your tank. Also, I recommend you NEVER put the water from the store into your aquarium if you can help it. It would be great if we could trust everything we purchase, however you just simply do not always know the conditions of the water. You may not be able to see anything wrong, but the bag water could transfer floating organisms and parasites into your system that could lead to illness. So, don't chance it. Pour the new specimen into a net from the bag, and from the net into the aquarium. There are situations where this may not be possible with some fish or corals, in those cases try to transfer into a neutral container of aquarium water, leaving as much of the bag water behind as you can.
So, the first step in acclimation is floating the closed bag in your tank for a few minutes to allow the water in the bag to reach the same temperature as your tank. Now, for some of your hardier specimens this may be all they need. However, some of your more sensitive fish and especially corals may need more than temperature adjustment. This requires the second acclimation step of emptying 1/4 - 1/2 of the bag water, and replacing it with your tank water. Allow the specimen to now float in this water for about 10 minutes. This should be all you need to do for most fish, but some corals may require a little more care. After temperature and water acclimation, placement of new corals can be extremely important. Always refer to their lighting requirements if you are not sure. However, keep in mind that if the tank lights at the store are not as bright as yours you may need to adjust the placement of the coral over time from low to high to prevent burning. If the store lights were brighter, but you want your coral lower in your tank, you may need to start it high and slowly move it lower to allow it to adjust. This is a hobby of many scenarios and many methods. For now, if you stick with these basic ideas, you should be fine.