Hey Jaff,
We had problems with various coolants not mixing well with our very hard tap water. Synthetics would go bad quickly (totally stunk up the place), de-emulsify, and rust our mill tables. We tried using water purified by a reverse osmosis filter, but it only fixed the problem temporarily. Coolant would still go bad eventually.
Then we started using Master Chemical Trim 585, a semi-synthetic, and haven't had any problems since. Works great on everything including Titanium. We threw away our aquarium airstones and put all the air pumps in a box for storage. We simply don't need them anymore, and we don't even use the RO filter when replenishing our tanks. There are some equally good coolants out there from other brands, but Trim 585 is what works for us. Whichever brand you decide on, semi-synthetics are the way to go.
Also, the next time you switch out your coolant, consider adding a cleaner to your old coolant before draining it. Master Chemical makes a product called Whamex that cleanses your machine nicely. Add a gallon or two to your tank, run the coolant system overnight, and any crud or microbes will be loosened/dead by morning. Drain the tank entirely, using a shopvac to get every last drop, and then fill with the new coolant.
-Sol
Glacern Machine Tools
www.glacern.com