It's hard to say how long the FC was zero. But it was hours (one time). Also, I do remember the walls feeling a bit slimy at one time after that. I thought the acrylic was a little slimy because of the recently added Borate (recommended by Nitro for softening the water) so I didn't pay it particular attention, but I did notice it. Tub seems normal now after a zap of high Chlorine over the weekend, but I'm taking no chances. New filters and Nature 2 are here and I'm doing a water change this weekend (water 4-5 months old but always clear).
What sucks.........If I miss just one day checking for FC (say I'm sick) I may contaminate the water, a bit frustrating.
Thanks for all the great advise and links!
Wish me luck on a grow-back......