From the results you get, I cannot really say its CO2 overdose but the level is slightly on the high side. 28ppm is to me on the high side but I think Bclee has 30-35ppm for his tank and with no troubles except for introduction of shrimps? Maybe you can try cutting back on CO2 to reach a pH of 7 to see if it helps. Other question, does your office's aircon go "off" after office hrs? If it does, then probably the large temperature swing affects the shrimps faster than the fishes.
Plant Physiology by Taiz and Zeiger
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