Raj, I agree that your cure is basically moving the fish from your OTS DT to your HT, not the erythro. Your tank is not overstocked, but I am curious, what is your cleaning regimen, especially your substrate. In addition how is your sump set up? Mechanical filters need to be cleaned/replaced frequently or they become nitrate generators contributing to OTS.

Also this statement is confusing - ph _____ 7 early in the morning (off gassed) going down to 6.5 by the time the lights go out 3:15 PM. In a low tech AKA no CO2 tank the pH will be lower in the AM as there will be a higher CO2 content as the plants are net O2 consumers in the dark and then the pH goes up as the plants consume dissolved CO2 during the day.

Finally how are you dealing with chloramine?