This is something you are not ...

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This is something you are not going to get to the bottom of.  Even if the op was doing something untoward to get something in a well mixed system to attack one set of diffusers and leave the other unscathed is difficult.  For all you know it was down to a manufacturing fault in a single batch which were fitted in this one tank.  Replace them, get the process back up and running and move on.