This may be caused by a Parasitic attack, or poor water quality [especially pH crash] which can cause this and you should, if possible attempt to lift the pH - this can be achieved quite successfully by adding a quantity of Oyster Shell into the filtration system - there are also other pH lifters available.

So Check your water quality first, and if the pH seems to be stable and within the permitted perameters then try treating with Salt first as described elsewhere on our treatments.

If you are sure your water quality is fine then it may be necessary to take a scrape and examine this under for parasites under a good microscope if you have one available .. once the identiy the parasite is known this will then allow you to treat for that particular visitor.

If the tear is not so bad then the best way is to simply apply a good topical dressing such as Koi Doc or any other good proprietary brand that you can trust. One treatment that I have found to be extremely effective is Silver Nitrate /... this is available from Veterinary Surgeries and perhaps some of the better privately owned Chemists on the high street - this is best applied using the specially prepared Silver Nitrate sticks, that resemble a long match with the head being the Silver Nitrate treatment ... this should be dipped into a something like an egg cup of warm, not hot, water to soften it for a split second ... and then applied to the spit fin along each edge .. Care when using these Silver Nitrate applications, so that you do not get it on your hands or even the work top you may be working on .. as it will badly stain whatever it touches ... even though initially the stain will not show - but 5 minutes later - oh dear !