All fish will tend to 'Flash and Scratch' if they have an infestation of parasites .. in the same manner if we have an itch we tend to scratch it.. There are many parasites which cause this 'Flashing' one that particularly seems to bother the fish more than other parasites is 'Trichodina, which when viewed through a microscope seem to appear like small 'flying saucers' .. the treatment for Trichodina is Potassium Permanganate .. at the rate of 1.5 gram per ton (220 gallons)..

But almost any parasite can cause fish to flash or scratch on things the problem is that the parasites damage the skin, opening it to infection, another problem is that the fish damage their own skin when they flash or rub themselves on an object, so treatment for the parasites causing the fish to act like this is important if you do not wish to have damaged fish that will cause you more headaches treating those wounds.

Another cause for this 'flashing' can be changes in the water quality. Large freshwater changes can, as we know, cause flashing as the fishes' skin adapts to a either slightly higher or lower pH levels. Chlorine or perhaps metals in new tap water can cause flashing, so again I stress how important it is to use a good water purification system on your pond.

But before making any decision as to what it is that is causing your fish to flash or scratch .. you must have the facilities available to take a scrape and to examine that under a high power microscope .. If you don't have such a microscope then may I suggest that you try to obtain on you will find that it is money well spent.. failing that then try to persuade a friend or colleague who you know has one to either lend you it or to come round and us it for you. 

Also, and the manufacturers won't like me saying this, but certain black plastic pond liners which can contain large amounts of UV stabilizers and elasticsers can be the reason for irritation sufficient to cause excessive flashing.