Are you doing a continuous nannochroposis culture? If a microscope is availble, look and see if there’s anything else swimming in the culture besides nannochroposis. Sometimes culture contamination can occur. You probably have dead phyto mixed with live phytos in the culture now. I recommend pouring it in a clean 2 liter bottle and let it settle overnight, harvest whatever is still suspended. If you inspect these under the microscope, they are still live and should be motile and should be good to start a brand new culture with then feed the bottomed out phyto to your tank. I snap photos of my culture daily and compare the photos based on culture coloration on the phone and can usually tell the whether the of nutrients/fertilizers is depleted or not. If same color over a 24 hrs period and no visual change in color darkness (culture density), then it’s safe to say that culture is good to feed the tank. Hope this helps save your culture.
View attachment 171401Happy reefing!