i think PH swing was a myth, even if it was real, i don't think many people seen their fish died from it. they however killed them through overdose of co2.
I suppose it very well could be a myth. I never tried the 100% RO/DI because I was told by everyone you would get huge ph swings due to lack of TDS's (Total Dissolved Solids). Without buffers, it would make sense that there would be a more unstable ph. Question: would the co2 cause a larger ph swing in straight RO water than water that had been reconsituted? And is co2 just way more effective in really soft water? Is that necessarily good for fish? And, is that good for the plants?