Hey Marc,
I have been using the Miracle Gro also for my phyto cultures, and I agree, it is almost impossible to have a crash using this stuff...but will crash if left for extended periods. I experimented, because I was so surprised my cultures just kept going and going and going with this stuff. Well I left one bottle just to see it crash, and it lasted almost 3 months before it finally crashed.
One thing I do notice though, using the Miracle Grow as a food source for the culture, is that the cultures have an extreme high concentration of Po4. I am afraid to add any unnecessary Po4 to my tank. This is how I now know when to harvest my cultures. I test the cultures for Po4 from week to week with a quick/easy SeaChem test kit..as soon as the Po4 level gets below 0.5, I know my food source is getting low and my cultures are ready to harvest and dose. Have you ever tested your cultures for Po4?