An important thing to consider is whether the fish you want to buy is compatible with the fish and coral you already have. The diet, aggression and territoriality of your fish all need to be considered before mixing different species of fish.
You should be aware and of the aggressiveness of the fish you have and the fish you plan to introduce. Only add fish that will be able to compete with the fish that you have but will not overpower them. If you add a fish that is too aggressive it is likely to act boisterously, eat all the fish food and attack and bully the other fish in the tank. It may even kill them. If you see this type of activity, remove the trouble-maker before it is able to create problems in the tank.
Some fish are not aggressive, but are territorial. An aggressive fish will attack for seemingly no reason other than to be dominant, while a territorial fish will drive fish out of its territory but will leave them in peace when the fish are out of its territory. The size of the territory required by your fish will determine whether they are a threat to other fish. Some fish, for example the Dottyback, are territorial but can often be housed safely with many fish because the territory they require is small, leaving plenty of room in which the other fish in the tank can safely swim. On the other hand, fish like Coral Trout can get so large that their territory can be the whole tank.
Predatory fish are again different from territorial and aggressive fish. Predatory fish are primarily concerned with feeding. They will eat whatever will fit in their mouth. An example of a predatory fish if the Lionfish. This fish is not aggressive or territorial but it will eat any fish that will fit in its huge mouth, which it can open to be about the same size as its body. When selecting fish it is safe to assume that all fish are predatory, so always make sure that the smallest fish in the tank cannot fit into the mouth of the largest fish.
For certain fish, timing will also matter. Most groups of fish are best added at the same time to reduce territoriality. For example any Tangs should be added together, any Clown Fish should be added together. This is also true for many other fish including Wrasse and Angels. Other fish live in large schools in the wild and tend to fret when kept in aquariums singly or in small groups. These fish are used to having a lot of their own kind around them as an instinctual form of security. When they are placed in aquariums that are lightly stocked with fish they get stressed thinking there is danger because the rest of their school is absent. This can be the case when all the other fish appear fine but an individual seems to be jumpy and breathing quickly. Some examples of these fish are Blue Tangs, Green Chromis and Anthias.
Most fish take about 3 days to settle into a new tank. In this time it is common for them not to feed and act abnormally. It is important to monitor new fish extra carefully for the first week for stress, behaviour, aggression and feeding habits. Always watch for changes in behaviour and act when it is required.
If you are going to introduce fish to a tank with Coral and Invertebrates it is important to identify which are likely to be a threat to them. This could be identified as A, B C fish. Some fish e.g. C fish will eat coral like Butterfly fish and Angelfish while others e.g. B fish will nip at it sometimes like Triggerfish, Pufferfish, Foxfaces, Rabbitfish most Wrasse, most Tangs, Moorish Idols. Others are mostly safe with coral e.g. A fish like Damsels, Clownfish, Dottyback, Blennies, Cardinal fish, Lionfish, Anthias. Filefish, Pipefish and Gobies.
Your wish list is a way of reducing the risk of introducing a coral eater to a coral tank. If you get a fish that only nips a particular type of coral you can consider avoiding that piece of coral in the future. Hungry fish are more likely to graze on coral, yet even if your fish is not eating the coral it may stress it by nipping it regularly which caused it to often close, depriving it of light.
Most fish need similar amounts of nutrients but the way they process the food means that not all foods are appropriate for all fish. You must consider what you will be feeding that tank when you buy fish for the tank. Herbivores have long digestive systems and a designed for consuming green foodstuffs while predators have a short digestive system for eating fish meat. Some high quality pellet and flake foods can be fed to all fish but many natural food like fresh fish and seaweed are not suitable for some fish e.g. Meat is not good for herbivores and seaweed is not good for predators, this is because of the way that the fish process that food.
It is very important not to introduce new fish unless that your aquarium is running perfectly for the last month. You will increase the chances of problems like Whitespot if the fish are introduced more regularly than once a month, so add some fish then wait a month then add some more then wait a month. It will also help to ensure that you are running a lower salt level e.g. 1.020 when you introduce new fish as this will help lower osmotic stress on the fish and also help reduce the chance of problems like Whitespot.
A quarantine tank is a very good idea to help protect your display tank from disease outbreak.
Regardless of where you are at it is worth asking the right person the right questions to ensure that the fish you add are likely to work together. Regardless of how qualified the advice you get, fish are fish and in the end they do what they want how they want. Just because 10 experts tell you something is likely to happen, it doesn’t mean that that will happen. Understand that the fish you buy are your responsibility and your responsibility alone. So if the fish you buy don’t seem to be mixing well, it is up to you to separate them, before too many fish are affected by the troubles.
The key is just as much in the monitoring and the action you take once a problem is identified as it is in initial selection.
Good luck and enjoy Paul Talbot