I agree. That is a pretty big bioload for a 13.5-gallon tank also. Plus it also depends on the source water because I know diatoms feed off of silicates which could be a sign of bad source water. Are you using a rodi system?Did you start the tank from new ? If so then I can’t see the tank being cycled in a few days. Diatoms will go eventually. How often do your water changes ?