Before harvesting the nano I check the culture vessel. If there is any kind of residue or "gunk" in the container I will move the nano to a new sterilized vessel, running the greenwater through a filter cloth first to catch debris. As for the crashes, the nano goes pale green and foams a lot, and then it turns yellow with a bunch of gunk settled on the bottom. I have tried adjusting air flow but it makes no difference. Something that I have noticed with the greenwater used for rotifers,, it doesn't turn to the color of tea,, it looks like rusty water, taking on a cloudy orange appearance. The rotifers do fine in the greenwater though.