Plan additions to your tank before getting to the store with cash in hand. Or even better, plan out the tank (with options) before adding anything.
Get some books, read online magazines/forums, ask advice, invite criticism, and develop a plan for the tank. First thing to figure out is what kind of tank you're hoping to build. Do you want a predator tank? A biotope tank? A peaceful community tank? An SPS tank? Something else entirely? There are a lot of good reasons why you might not have an answer to this first question, but you should come to some decision before making any other decision as it will dictate just about everything else about your tank.
If all you want is a "reef tank" and you really can't be more precise than that, then you want a community tank and the only question that remains is how much excitement (predation) you want going on in your tank. Me, I'm for peaceful cohabitation, but other reefers don't seem to mind the possibility of having some excitement in their tanks. That excitement will almost certainly be measured in the loss of a few fish, so if that's what you want, be aware of what you're getting.
Once you've got a top-level goal for your system, other decisions become a lot easier. Presumably you've already got the tank running, so the size is fixed. Start asking and reading. What you're looking for are ways to fit animals into your tank, much like putting pieces into a puzzle. You can't put too many fish, or fish that require too much space in the small volume of a tank. You also probably don't want to put fish that will agressively compete for the same territory or the same foods.
For my community tank, I wanted a few commensal relationships, so I decided on an anemone/clownfish pairand a shrimp goby/pistol shrimp pair. Both of these pairs tend to stay close to one location and close to the bottom of the tank, so if I put one on each end of the tank, they wouldn't bother each other. This leaves the mid/high water region of the tank pretty much open. A small school of smaller chromis would be fairly peaceful, active, and tend to stay higher in the tank, out of the way of the clownfish and the goby. For a moderate tank size (up to 80 gallons), this is probably it (rule of thumb: 1" fish/5gal water). For something bigger, maybe a royal gramma to live in the rocks or attempt to create some mating opportunities for the goby or the clownfish...
Now, note something about my plan. It isn't too precise. My plan is not to pair a Stonogobiops yasha (whiteray goby) with a Alpheus randalli (pistol shrimp) and then to pair a Amphiprion ocellaris (false percula clownfish) with a Stichodactyla mertensii (carpet anemone) and then to add a school of Chromis vanderbilti (Vanderbilt's chromis). That may be the most exotic and beautiful combination I could possibly go for, but it's unlikely that I'm going to find exactly what I need in the timeframe that I need it. Patience is indeed the primary virtue of the aquarist, but I'm not waiting around six months for someone to get around to catching a few hard-to-catch chromis when other varieties are so commonly available.
You'll still need to read up on all of the features of the animals you're planning, otherwise you might not know to add the goby/shrimp pair first, followed by the anenome/clownfish pair, and then later, the school of chromis (in increasing order of activity so that each fish has the best chance of settling in). Also, you may find out other facts like mixing oceans (the royal gramma is Atlantic/Carribean, the others are Indo-Pacific) can increase the risk of disease in your tank...
So once you've got a stocking plan (the fish you want with the best timing to get each new arrival), the last piece of advice is to take your money in hand and go into the store with as much information as possible. Know exactly what the fish you want looks like (stores are rather famous for confidently attaching wildly inaccurate names to the fish they're selling), if there are any similar-looking fish (and how to tell them apart), how this fish looks when healthy (usually meatier is better, though a bulging belly is almost always bad), and don't accept last minute substitutes unless you're sure the substitute is acceptable (I would accept a school of cyan chromis if someone had just bought all of the green chromis, but I would not be okay with a few blue damselfish, no matter how closely related they were to a chromis (LFS actually said this :evil: )).
Above all, be patient. If they don't have what you want this week, someone will have it soon enough. Keep your money and spend it on your terms.
Regards,
Ross
