In general, fungus, a slimy, snotty, looking mass that may or may not have a mild yeasty odor is more of a problem for acid fountain solutions than for neutrals. Bacteria infestations, invisible, but so smelly the neighbors complain, are more common in neutrals than in acids. Some people get infested with both!
Best protection is to keep the solution circulating. Both fungus and bacteria are usually introduced into the system through contact with the air and if the fountain solution isn't moving, only the bacteria/funguscide ingredients at the surface (a very small proportion) have any effect.
Second line of defense is bleach. It indiscriminately kills all of these organisms and is inexpensive, available, and can be used to whiten your whites if need be.
