I have discovered your problem.
You are putting CO2 tablets in your reservoir? Noooooes!!! Carbonic acid FAIL!
Quick quick go check adn adjust your res pH to 5.8 if it isn't there, stop with the CO2 tabs, and also post up the value of the runoff pH coming off your slab before it goes back into the res. Those two numbers- res and runoff- are going to combine to tell you what's up and WHERE the source of the problem is physically located.