I really advice you to get a koi dealer
to come out and look.
If you do not know much about koi or water it is going to be dangerous to take advice from people and try and do it yourself. Rather get a expert out and if you can afford it let him do the treatment until you yourself have enough knowledge about keeping koi.
It sounds like you might have water problems or parasite problems, just like humans koi suffer from diseases and need treatments from time to time. There is no miracle bottle of something you can put in to cure them of all there problems - i have seen pet shops selling these miracle cures that work for all parasites and diseases.. The problem needs to be identified and then that problem needs to be treated that i why i suggest a koi dealer - he is like a docter.
Also remember that koi live in the water, if the water and filters are dirty or not looked after it is the same as a person living in a dirty house with a toilet that has not been flushed in months, diseases will break out.
Try Cape Koi Aqaurium, they are in joostenberg vlakte. http://www.capekoiaquarium.com/
Do not try and explain to them what is happening, get them to come out. It will cost you a call out but its better than guessing what is wrong.
I am sure they will be able to help you but if you want to save some money and keep the koi healthy in the long run you will have to learn how to look after koi.
Maybe join SAKKS also know as KoiSA http://www.koisa.co.za
and subscribe to their magazine, that will teach you many things regarding koi and we have an active community in the cape.