ugh, three posts in a row. Put the soap box away adam.

The last thing I'd say about the real risk to 100% water changes (assuming that it's your only method of maintaining a tank) is not that it can take something like dKH from 6 to 8, it's that between the water changes, your dKH is allowed to fall from 6 to 8. It's the ongoing, day to day stability that matters, and if over a week your levels are fluctuating from very low to very high, or very high to very low and you're doing nothing in between to keep them 'stabile', you're either not doing your 100% water changes regularly enough, or you're not doing enough in between the water changes to keep things stable.