If you have been keeping fish for any length of time I'm sure you already know the importance of quarantine. You may have learned from others in the hobby who are more experienced or you may have learned the hard way like myself. I lost some of my favorite fish by adding a new fish to an already established healthy tank. I made the mistake of thinking that the newly acquired fish was healthy simply by looking at it and observing it for a short while in the sale tank. I could not have been more wrong.
For this reason observation is important when contemplating buying a new fish. You should be looking at all the fish in the sale tank and not just the one you want. If you see dead fish in the tank or fish that are covered with ich (aka white spot) then this is when you want to walk away. If everything checks out and you do decide to make the purchase you still are not in the clear. This is where the quarantine part comes in to play.
Quarantine tanks (or hospital tanks as some call them) are simply a tank or any type of water holding container that you can use to isolate your fish. It will give the new fish (or sick fish) a place to recover from the stress of moving them and it also allows them to acclimate to their new home as stress free as possible. It gives you a controlled place to observe, feed and treat the fish if necessary.
New fish should be in quarantine a minimum of four weeks but even better if you can leave them for eight weeks. Certain illness or parasites can take a little while to show themselves. Watch your fish for any signs of abnormal behavior such as clamped fins, bottom sitting for long periods of time, listlessness, gasping at the surface, lack of interest in food. These are just to name a few.
If you have a microscope now would be a good time to use it. If you see any parasites you can identify them and treat with the proper medication.
Don't bother scraping a fish that is dead as any parasites that may have been on the fish will quickly leave once the host is dead. If you are unsure about using a microscope Dave54 has a very informative thread with some great pictures that will help you through the process. You can find it here: http://www.gardenpondforum.com/threads/taking-a-scrape-of-your-koi-or-goldfish.14555/
Quarantine or hospital tanks don't need to be expensive or even really large. In fact, it's preferred that they are smaller (obviously depending on the size of the fish) as it will be easier to do water changes and also cheaper to medicate the smaller amount of water. Remember that the fish will only be in quarantine temporarily so don't fret over the size of the tank.
The tank can be something as simple as a plain 10 or 20 gallon aquarium or even a Rubbermaid storage bin (when not in use makes a good place to store extra fishy items) If you are setting up for a large Koi you will probably want to use a Rubbermaid stock tank or something similar of at least 100 gallons or more.
A cover for the top of the tank should be used although it's not necessary to have a light over the tank. If using a stock tank a pond net would suffice. The fish may even appreciate not being under bright lights while acclimating or recovering from illness. Add something for the fish to make it feel more comfortable such as a clay pot or some type of cave for them to hide in (don't use live plants in quarantine as you may need to medicate which would not be
good for a live plants) keep the tank simple by not using gravel or stones on bottom of the tank as it will be easier to keep the tank clean and observe that way.
You should use some type of air pump for aeration appropriate to your quarantine set up. You may also want a heater (trying to keep temperature somewhere between 72-78 degrees Fahrenheit) and a thermometer to check the temperature.
A filter is not necessary if you're performing daily water changes and/or medicating your new fish (also do not use carbon in the filter if you are using medication as it will absorb it) I normally keep a spare sponge filter running in one of my aquariums so that it is matured and ready to be used if necessary. If you have a larger quarantine set up you will want another type of filter that can handle the size of the tank as well as possibly a UV. If you choose to use the UV make sure to shut it off if you are using any type of medication. If using medication it is important to know the exact size in gallons of your quarantine tank. Makes it safer to medication so that you don't under or over dose your fish.
Goldfish and Koi seem to do better with another fish in the tank. Not exactly sure why but maybe it's their shoaling instinct? So when buying a new fish if possible buy two. And if you don't have room for two then you may want to borrow a fish from your pond to use as a buddy to your new fish.
Once you are sure your new fish is healthy it's time to move him to his new home.
You may want to do a water change in the established tank or pond prior to adding the new fish. Check water parameters (ammonia, ph, nitrite, nitrate, and temperature) and if all checks out well you are good to go.
I like to move my fish using my hands but if you are not comfortable with that you can use a net. This should be done carefully with as little stress to the fish as possible so do it slowly leading the fish to the surface rather than chase it around the tank. You may want to use two nets.
You can guide the fish with one and into the other.
Always remember to disinfect all equipment including nets after each use. Be careful not to add water from quarantine tank to the established tank or pond.
By following the simple rule of quarantine you will enjoy your fish for many years to come.