I show you the process and materials needed to flush/descale a Rheem/Richmond tankless water heater. These instruction should work for most other brands of tankless water heaters as well.

The Everbilt pump I used: https://www.homedepot.com/p/Everbilt-1-6-HP-Plastic-Submersible-Utility-Pump-SUP54-HD/300534069