Some fish will turn your aquarium into a beautiful and peaceful biosphere while others into a chaotic boxing ring.
There are many factors to consider when selecting fish. You need to ensure that you have the experience to look after the fish you select, that the fish you buy are healthy, and that they are compatible with the fish you currently have.
As a starting point, it is very import that you only buy very healthy fish because the best way to keep strong healthy fish is to start with strong healthy fish. Buying a fish because it doesn't look well or happy and you feel sorry for it and want to give it a better home in your display tank is not a very good idea. An unwell fish can introduce diseases into your tank that can infect your other fish, and may even cause them to die. It helps to become familiar with a species of fish before you buy it as this will allow you to be clear on exactly what it should look and act like. Ensure the fish looks alert with clean clear eyes, fins and scales. It is also important that the fish appears eager to feed and can maintain its position in the water column. And finally, as a precaution, only buy from a clean healthy store that you trust.
To break this down I simply refer to all fish as number 1, number 2, or number 3 fish. I refer to number 1 fish as fish that most people can easily keep in regular aquarium conditions. Number 2 fish are fish that from my experience work for some people and not for others and number 3 fish being those which don't work for most people.
Some examples of number 1 fish are Damsels, Clownfish, Dottyback, Triggerfish, Pufferfish, Foxface, Rabbitfish, most Wrasse, most Tangs, Blennies, Cardinal fish, lionfish. Some examples of number 2 fish are , most Tangs, Boxfish, Angels, Gobies, Sweetlip. Some examples of number 3 fish are Moorish idol, Powder Blue Tang, Achilles Tangs, Anthias, Filefish, pipefish, Mandarin fish, Butterfly fish.
Many people select marine fish by wondering into aquarium shores and looking around until they see a fish that catches their attention at that time, they will them ask the staff member closes to them if this fish will go with the few of their fish that they remember to name. If the staff member says yes then that is a green light to buy the fish. This approach takes very little into consideration and will as often as not result in the person purchasing a fish that was not likely to work from the start.
After a considerable amount of time and money, the person will start to understand which fish work in their aquarium, mind you most will have given up marine fish before this time has arisen. Instead of the impulse approach where you slowly learn the hard lesions of fish selection, I strongly recommend starting with a wish list.
A wish list is simply a list of fish that you wish to keep together in your tank. The beauty of a wish list is that you are able to show it to other experience aquarists to get their opinions on how these fish are likely to go together. If you have fish already you can add them to the top of the wish list to reduce the chance of adding other fish that won't work with the fish you have. If you have a wish list you are likely to seek out experts to ask in order to gain the right advise. With the impulse approach you are far more likely to ask the nearest sales person and hope that they know. With the wish list you are able to use the opinions of a range of experts to save you a lot of time and money learning hard lesson on paper instead of with real fish. This is a very responsible and economical approach.
When selecting fish for your aquarium there are several things to consider before purchasing it e.g. diet, aggression, territoriality and weather it will nip at your corals.
A simple thing that you need to remember is that fish don't want to die. They will only die if you don't provide them with at least their basic minimum requirements. By researching a fishes basic minimum requirements first and asking a few people for their experiences keeping that fish you can massively increase the amount of success that you have when keeping marine fish.
The use of the wish list is going to help you make sure you are mixing fish that will commonly work together. 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 affected by the troubles.
It is important consider aggression when selecting fish. Monitor the aggression of the fish you keep and only add fish that will be able to compete with the fish that you have and not over compete. If you add a fish that is too aggressive for the fish you have it is likely to act boisterously and eat all the fish food and attack the other fish in the tank, even killing them. When you see this type of activity remove the trouble maker before it is able to cause you any more problems. 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.
Some fish are more so territorial then plan aggressive, an aggressive fish will attack fish for seemingly no reason. A territorial fish will drive fish out of their territory but leave them when the fish is out of its territory. It is worth considering territorial behaviour when selecting fish. Some fish like the Dottyback is territorial and can often be housed safely with many fish because its territory is small, leaving room for the other fish in the tank, while some other fish like coral trout can get so large that its territory can be the whole tank.
There is a big difference between territorial, aggressive and predatory. Territorial fish drive fish out of their territory, aggressive fish attack other fish for what can seem like to reason but to show dominance and predatorily fish eat other fish. Predatorily fish don't have to be aggressive or territorial. Predatorily fish are primarily concerned with their belly, what can fit in their mouth is what they will eat. A perfect 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 is about the same size as it body. When selecting fish it is worth assuming that all fish are predatory.
When creating you wish list also include when you intend to adding the fish, because you also want to gather comments on this. Most groups of fish are best added at the some time to reduce territorially e.g. any tangs should be added together, any clown should be added together, this is also true for many fish even Wrasse and angels.
Many fish take about 3 day to settle into a new tank, in this time it is common for them not to feed and they may act differently. 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, action must be take when it is required.
Some fish live in large schools in the wild and do tend to fret with 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 lightly stocked with fish they stress 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.
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, Foxface, Rabbitfish most Wrasse, most Tangs Moorish idol. 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 reduce 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 in the future. Hungry fish are more likely to graze on coral , even if your fish is not eating the coral it may stress it by nipping it regularly coursing it by often closed, 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 food stuffs 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, this will help lower osmotic stress on the fish and also help reduce the chance of problems like Whitespot.