I only buy the 100 oz. size bladders as I tend to go through a lot of fluid. I usually pour a 64 oz. Poweraid/Gatoraid bottle in, and then top off with water (kills off that sticky sweetness). If it is expected to be a particularly hot ride I'll trim that back to a ~50/50 mix.
I have frozen a partly consumed bladder of my normal mixture when I didn't have time to immediately clean it. The only times that I've had growth is when I forget to do something, anything, with the bladder. I've had to toss a couple as I didn't think that I'd get them sanitary enough. My normal rinse is to first drain the bladder, then fill with hot water and drain at least 1/2 of the bladder through the bite valve before removing it.
I used to swerve around my hallucinations, now I drive right through them.