My pond hasn't gone red, but it didn't smell right last tuesday when it was very hot and the water wasn't very clear as it usually is, so I cleaned the filters and done a slow water change overnight and seems ok now.
I think in my case it was due to the sudden jump in temperature. It's been 15-20° for ages then jumped to 30°+ and I think this rapid jump in temperature has shocked the bacterial life in the box filter (which is in the sun most of the day) and possibly poisoned the water.
Sadly I lost a very big koi that I had for nearly 20yrs
Re the original question - was the water greenish before and was there any algae on the pebbles? If there was, then maybe the algae has died and caused the colour change.
Last edited: 27 July 2016 20:56:06