my koi

Question
Hi, I have a koi, not sure if it is or not but it has the shape of a goldfish but has barbels. Anyway I have 3 large koi in a 200 Litre fish tank. A platinum ogon (15cm), a benigoi (10cm) and the gold carp. The gold carp has a recurring red patch on its right side of the body, it looks like a rash of some sort, the scales point out. I don't like how it looks and i am worried for its welfare this has recurred multiple times, I tried placing it in a sick tank, it healed but a few days later the rash came back. I dont know what it is but some of my theories are my plecos, or algae eaters could be trying to eat its slime. I've also noticed my larger koi attacking the infected spot and making it worse. I do fortnightly water changes. Other fish in there are black ghost knife (they are in there temporarily it's summer here).

ANY HELP WOULD BE GREATLY APPRECIATED THANKS! :)

Answer
Hi Tim,

Thank you for your question.
The answer is not easy, however I will try.
A 200 liter tank is somewhat small as the koi are general counted as 1 koi per 1000 liters.
The recurring injury can be of different types.
If it reoccurs it can be parasites in the water that will attach themselves to the skin.
It will infect the fish. Symptoms can be jumping, scratching / flashing. It then also tends to infect other fish. It can be bacterial or because of poor waterconditions and stress.Best way to go is to test the water, take some scrapes of mucus and watch trough a microscope.
This will eliminate all guesses and give a strait answer.

Good luck and please keep me informed if possible.

Kind regards,
Raf