Two frequent posts on marine aquarium based forums are along the lines of:

In most cases, the new fish has been placed directly into the main display tank and then things go downhill from there. These two common problems and others can be solved by first placing all new fish into a quarantine tank.

Why will quarantine help? I believe there are two main reasons to quarantine ALL new fish:

Acclimation

While the second reason is the one that comes to most people's minds first, I'd like to spend some time on the other as I feel this is more important.

Consider what happens to a fish you buy from the time it is caught until the time you bring it home.

The time, from capture to the LFS tank, may be 2 days to over 2 weeks and in this time it is unlikely that the fish has been fed.

At the LFS, the fish may get some food, but as the LFS has many fish to worry about, this fish won't get any special treatment. As the fish was wild caught, it won't be accustomed to eating prepared foods and may not even take food that is floating.

You are now on your way home from the LFS with a fish that has been highly stressed for anywhere between days and weeks. It probably hasn't eaten and is likely to be weak.

What's going to happen if it is placed in your main display tank with a bunch of healthy fish that will eat anything dropped in front of them?

More than likely it will hide. It probably won't get much of the food added to the tank as the other fish will get the food first. If it needs "training" with live foods, it will have to compete with the existing fish. There is also a real danger that the existing fish will harass it or even fight with it.

The alternative is to place this new fish into a tank on its own. The tank would have sufficient hiding places and you could dim the lights to lower stress. Most importantly, you can give the fish one-on-one attention to get it feeding and this allows you to try a wide range of foods. Finally, once the fish is feeding, it has a chance to build up strength and get used to living in captivity.

After 4-6 weeks in a tank on its own it should be feeding on a variety of foods. It should have regained any weight it lost between capture and your tank. It may even start to grow. Most importantly, it will be under very little, if any stress.

Disease prevention

Disease prevention is another good reason to quarantine.

Fish may carry with them diseases from the wild or may come in contact with infection in holding tanks or at the LFS. The most commonly encountered disease is marine "Ich" which is caused by the protozoan Cryptocaryon. The life cycle of Cryptocaryon is 2-3 weeks, so isolating a fish for 4-6 weeks greatly increases the chances of detecting the infection before the fish is introduced to the main tank where the infection could spread to the other inhabitants. If disease is detected during the quarantine period, it can be more easily treated in a smaller tank without concerns for the effects that medications will have on invertebrates.

There are other and more severe diseases such as Amyloodinium which are not only highly contagious, but are very quick acting. These must be treated immediately for there to be any chance of saving the infected fish. Having the fish in a separate tank means that treatment can be administered immediately without stressing the fish any further.

If the water quality in the quarantine tank is good and the fish is exposed to little or no stress, diseases are uncommon and treatment is rarely necessary.

Why not quarantine?

Some people argue against a quarantine tank as they feel it adds more stress. This would only be true if the quarantine tank is not set up correctly. If there are insufficient hiding places or the water quality is not high, a quarantine tank may be more stressful. However, both of these problems are easily remedied.

How to set up a quarantine tank

A tank for quarantining fish does not have to be elaborate. It just needs to be a stable environment with good clean water and plenty of hiding places for fish.

A quarantine tank should be at least 2 feet in length. Anything smaller may cause stress to the new inhabitant(s). A standard 3 foot tank would be better and this provides enough space for most fish. Even tangs and juvenile angelfish will do fine in a tank of this size for the quarantine period. My quarantine tank is a standard 3 foot tank.

The tank will need a heater and minimal lighting. A single fluorescent tube should suffice. You can get by with a standard daylight tube, and there is no need to use an expensive aquarium type tube.

My quarantine tank. It is 36" x 14" x 18" with shell grit and an air-driven DIY undergravel filter. Hiding places are rocks, coral skeletons and shells. I cannot treat with copper in this tank but to date it has not been necessary. I have used hyposalinity treatment with success.

Careful thought must be put into the tank decorations. Calcareous materials should be avoided as calcium carbonate absorbs copper. This causes two problems if treatment with copper is necessary:

PVC piping and ceramic pots make good hiding places, and they can be easily removed when it comes time to catch the fish. Live rock should not be used, as these are coral rubble which is composed largely of calcium carbonate. Additionally, you do not want or need invertebrates in a quarantine tank as these will restrict treatment.

As most wrasse species bury in sand at night, it may be necessary to provide sand in the tank. Silica sands and non-calcareous river sands should be used if you plan to keep wrasses.

A quarantine tank needs some form of biological filtration. The simplest biological filter is a canister filter with bio-balls. Another possibility is an under gravel filter using non-calcareous sand. The tank should be cycled in the normal way, and should not be used for quarantine until the ammonia and nitrite levels are both undetectable. Under normal circumstances, this should take 4-6 weeks. If the quarantine tank is set up at the same time as the main display tank, it will have cycled at some point before you would be putting any fish into the main tank.

The temperature and salinity of the quarantine tank should be maintained at the same levels as your display tank. This should be the same as natural seawater in reef environments: 26-29 C (79-84 F) and 35 ppt.

Quarantining Fish

The process to quarantine new fish is very simple.

The new fish should be acclimated as you would any fish. First, the bag is floated in the water to allow the water temperature in the bag to meet that of the tank. Ten minutes should be plenty of time. Next, gradually add tank water to the bag. Continually add water until you have added around 4 times the original volume of water in the bag. Since fish are fairly tolerant of changes to water parameters, this should only take around 30 minutes. Drip acclimation is usually the simplest method to achieve this. Fill a 2L (2 quart) container with tank water and siphon water from the container to the bag using airline with a clamp or tap such that it drips quickly. This will provide a consistent progression from the bag water to the tank water.

Some people recommend discarding the water in the bag by pouring it through a net into a bucket. The fish is caught in the net and then dropped into the tank with almost no water from the LFS.

Personally, I don't see the small amount of LFS water being much of a problem and most pathogens would be carried on the fish rather than in the water. I'd prefer to subject the fish to as little stress as possible and so I allow it to swim out of the bag.

Once the fish is in the tank, it is a good idea to dim the lights if possible. Don't shut the lights off completely, as the fish will need to investigate its new environment and find suitable hiding places. Give the fish an hour or so to settle in before switching the lights off for the night.

On the following day, try to minimize your movements around the quarantine tank so as not to frighten the fish too much. Carefully observe the behavior of the fish. You will need to continue these observations for the time the fish is in the quarantine tank. Look for any abnormal behavior such as scratching and flaring that might suggest infection. A small amount of scratching is normal and some species will appear to scratch themselves on the sand when looking for food.

On the day after the fish is placed in the quarantine tank, you need to start to get it to feed. Some fish will start feeding almost immediately, however, others may be reluctant to feed or may not know that the food you provide is food. Be prepared with a number of different types of frozen foods. You may also need access to live foods such as Artemia naupii (baby brine shrimp) or adults. The movement will often pique interest and stimulate them to feed.

The fish should be left in the quarantine tank for at least 4 weeks. Six weeks gives more time for observation. A longer time may be necessary if the fish is not feeding well on a number of different foods or is not behaving normally. If the fish shows any signs of infection, the appropriate treatment should be administered.

If after 6 weeks, the fish is behaving normally, eating well and shows no sign of infection or loss of weight, it can be moved into the main display tank. The acclimation method above can also be used in the main tank.

Quarantine is a very important practice to ensure a successful marine tank with healthy fish. It ensures that any fish placed in your display tank is as fit as it can be and the most prepared for competition from the other fish in the tank. Additionally, quarantining fish will greatly reduce the probability of introducing disease into the display tank. A quarantine tank is very easy to set up and for the benefits it gives, it is definitely worth the cost and effort.