White spot dorsal fin, fish disease, infections

I have a large fish in my pond that got a large white spot behind the dorsal fin. I did a 30% water change. The fish loved it. But now, what do I do to get rid of the white spot?

Many fish problems can be treated with proprietary treatments from aquatic stores, but first of all there are other things to ensure to prevent this happening again or for the spread of the disease. Fish catch diseases when there is some other form of distress weakening their immunity. They are in fact usually secondary infections due to another cause.

Check your water quality.

If there is any imbalance try to do something about it. It may be a population problem, over feeding in cool conditions, over feeding in hot conditions, dare I say - a dirty pool, surely not! A chemical imbalance caused by concrete in or around the pool or even bad tap water perhaps creating a high pH, (A high pH exaggerates the effect of any ammonia or nitrites in the water. Nitrates are not too much to worry about.) Herons or other animals can also make life stressful for koi and goldfish.

Then treat the disease. If it just the one fish with the disease then you can treat in a hospitalising tank separately. In this way you can guarantee you get the dosage right. You can ensure the patient gets well fed with high quality food and the treatment chemical is not wasted. Oxygenate well as many treatment reduce the oxygen level.

If you decide to treat the whole pond then keep the fountain going. Do not mix treatments especially salt with any proprietary treatment.

Pond Salt at 3% or 3gms per litre in a bath for 10minutes at time should do it. This is a good all round bacterial remedy. Otherwise treat with a general bactericide

It doesn't sound like the classic 'white spot' or 'ich' which is caused by a parasite. (Look for twitching or scraping behaviour). If it is this it can easily be treated with a general parasite remedy that must be kept up for 7 days to kill off all the bugs at different stages of the life cycle. However it always best to treat for bacteria at the same time as treating for parasites since bacteria are the inevitable secondary infection after an attack.