Groundwater is the most suitable source of pathogen-free water. These sources are currently limited to both trout farms and hatcheries, and other dedicated aquaculture units. In most cases, surface water from rivers and canals is used as an inflow source. In such situations, the use of appropriate filters can partially reduce the number of stages of parasite entry into the influent. This is especially true for intensive cultivation in small reservoirs. These filters are usually preceded by a rod to separate coarse particles. Sand filters consist of a number of sedimentation compartments closed by fiber and sand filters. These types of filters primarily catch the heavier parasite stages that cannot actively move. Less efficient at eliminating mobile parasites such as Infusoria. Water from a pond with fish stock is completely unsuitable for these purposes. Chemical treatment of water supplies is an urgent solution, often with unwanted side effects. Disinfection of water entering fish farms using ultraviolet light is considered the easiest way to destroy viruses, bacteria and mold, but it is still not a common procedure. Influents from rivers and canals are slightly turbid and contain many suspended solids and dissolved compounds, which greatly reduces the disinfecting effectiveness of UV light in these conditions. Independent feeding without passing through individual ponds or reservoirs is very profitable. Water from each pond or reservoir should be drained separately and not flowed into other ponds or reservoirs. In particular, quarantine ponds and other reservoirs can be isolated in this way. Among other things, this principle implies transmission of pathogens through uncontrolled fish and spawning transport. In principle, the transportation of fish whose health status is unknown should be avoided. All fish transported must be accompanied by a veterinary certificate certifying that the fish was inspected prior to shipment, was healthy and came from an environment free of major infectious diseases. A list of these diseases is clearly indicated in the veterinary instructions. In addition to the internal research of each country, a list of diseases listed in the International Code is also mandatory for veterinary services. This list is currently being refined as diagnostic methods are developed and knowledge of individual fish diseases improves. Some viral and bacterial diseases can also be transmitted through egg laying. For this reason, their transport must be completed with the same veterinary certificate as the fish transport. Fish imported from other regions, whether native or exotic, must be quarantined for one year. The quarantine period can be extended. Up to 3 years for fish imported from abroad. Longer quarantine periods are especially important for spawners who are scheduled for further breeding of imported species. Livestock self-reliance on individual farms and similar organizations is an important way to prevent the spread of fish diseases. Only spa-inspected, disease-free fish that have been properly treated may be used in ponds or fish farms. Keeping juveniles from semi-artificial and artificial spawning systems out of contact with older age fish also minimizes the risk of infection. Preventing whitefish intrusion into ponds and fish farms is another important means of protecting stocks from pathogen transmission. These fish are the main source of ectoparasites and are especially dangerous during periods when the fish's resistance is low.