Sap or Saprolegnia is the main genus of water molds which can bring about significant fungal infections in fish and eggs. It's safe to say that all ponds have water molds in them. Unless the fish are compromised the slime coat will, in most cases, protect the fish from an invasion. Sap invades epidermal tissues In common with all molds, it does so by secreting digestive enzymes into the surrounding areas. Since enzymes in general break things down it's easy to understand that this secretion enables the fungi to more easily absorb nutrients from the affected area. SAP slowly is digesting the fish! at this point it is called saprolegniasis.
SAP can spread over the entire body, but is usually spotted as pale white or grey patches. Sometimes it will be green if algae has been caught in it. It is usually considered to be a secondary infection seen after damage to the fish skin or gills. However it can be primary as it is temperature dependant, occuring in cooler waters as a consequence of immune system shutdown.
SAP Treatment: It is generally agreed that Malachite Green is by far the most effective treatment. Be careful with this stuff! Eye and skin protection is required. A stock solution can be made with it and it is applied topically to thoroughly and deeply clean the wound. A dental adhesive may be applied afterward if desired.
Stock solution: 28.4 grams makes one gallon. To treat directly use the stock solution straight. For other treatments use one drop per gallon.
Treatment is should be made as soon as possible along with determining the actual cause of the invasion and dealing with that as well.