Sorry to hear about your struggles Daniel but glad things seem to be turning around. Not sure if you saw the thread from PDXmonkeyboy about his water related issues but he chased it down to an ammonia spike caused by increased chloramine in the water supply overwhelming his RODI and resulting in measurable ammonia getting added to his tank during water changes. Not sure if something like that could have happened to you (over a longer time frame) but the dates seem like they might align.
Sounds like you have some nice new fish lined up to join the DT crowd!