It seems like every koi website nowadays has got some kind of forum and the numbers are growing. They are a helpful source of information and advice so I've made a list of the biggest or which I consider the most authoratitive.
Infiltration Forum - Peter Waddington answers your koi related questions.
Koi-Bito - Japanese koi magazines' forum.
Koi Magazine Forum - Popular magazines' forum.
Koi.com.my - Malaysian forum with members from all over the world.
Koi UK - Maurice Coxs' British Koi Farm.
Richdon Koi-Help - Richdon Koi members only help forum - free to sign up.
Yorkshire Koi Society -A small community of knowlegable members.