Ack, your pH in your system goes to 6.0, but your tap water is 7.8 with Alkalinity of 100???? Something isn't adding up! What in your system is so acidic??? Check Hardness...if that is also near 100, your tap water is well buffered and something in your system is leaching a lot of acid to drop pH to 6.0 in just 2 days.
There may be dead/stagnant areas that are generating lots of hydrogen sulfide...your settling/solids tank?
