This year, I've had a hard time preventing red mold from growing in my Platypus hydration bladders. I don't know why mold started to be a problem this year, but I think I've finally found a way to clean it out of my bladders and prevent it from returning.
The secret is Efferdent, normally used as an anti-bacterial cleaner for dentures, but gentle enough for sterilizing the inside of platypus water bladders. To use, fill the hydration bladder up to about 2/3 of capacity and drop 1 Efferent tablet into the bladder for each Litre of water. After a bit, the tablets will start fizzing and turn the water in the bladders blue. The fizzing oxidizes any bacteria present in the bladder, killing and dissolving it. For maximum effect, tip the bladder over and make sure the blue solution touches all surfaces in the interior of the bladder. When the blue water turns clear in that classic Efferdent way, rinse the bladder out throughly with clean water, fold it up and recap until next use.