Straight distilled white vinegar, diluting it raises the pH and also does not allow you to save and reuse it. I strain the big chunks out with a paint strainer or some paper towels and pour it back in the bottle, the stuff lasts about a year before the pH rises enough to make it useless. Its only like $1.50 a gallon at WalMart and is an excellent safe cleaner.
By not cutting it with water it works faster and requires little if any brushing and cleaning, just rinse it under running water and almost everything falls off. My only caution is don't leave the rubber parts like O-rings and suction cups in too long or the rubber swells and later hardens, by too long I mean days though and the power head will usually be clean in an hour or two at most.