When you buy fish to add to your pond they are almost certainly from a different fish farm, or even country to your existing fish and they may have a variety of minor, or even major problems with parasites, or disease. It is for these reasons that you may consider it worthwhile to quarantine them for a few weeks to ensure that they are healthy.
To quarantine fish effectively a special pool must be built. It has to be big enough to cope with both the quantity and the size of the fish likely to be bought. It needs a good air supply to increase oxygen levels and ideally it should be heated to speed up treatment. The best place for such a pool is an unused garage where it will get extra protection from the elements. The water should be filtered in it and the filter must be mature and ready to cope with a sudden influx of fish. To do this it has to be kept running at all times and the pool must have some fish in it to provide food for the filter. The fish can be removed immediately prior to adding those to be quarantined and then replaced afterwards.
An Ultra Violet system added to the filter will help to reduce bacterial and parasitic problems, but all new fish should be dosed with anti parasite treatment as a matter of course. The fish should be closely observed for about a month and if necessary other treatments should be given. Pond salt is a very useful tonic in small amounts and a potent treatment for most problems in stronger doses.
After treatment the fish may be added to the main pool, but do keep an eye on them as problems could still develop.
Hopefully all fish bought from retailers have been quarantined when they were imported, or when the retailer acquired them, but there will always be some problems with newly bought fish. This is mainly due to the high stocking levels in the dealerís tanks and the constant stress that the fish live under in such conditions.