Water-cooled spindle maintenance

I have noticed the outlet water feed from my spindle is starting to get discoloured (brown) and i have read before its due to build up in the spindle. what's the best way to clean out the spindle/maintain it.

I have had the spindle in use for 1 year so far with a 20ltre tank with 4 parts deionised water and 2 parts coolant.

I'd be interested too as I've had mine now for nearly 3 years and ran it exclusively just on regular tap water but it doesn't have any of the above issues.