unknown substance in HW tank

I full time and replace the water heater anode rod once a year. I bought one of the Camco hot water tank rinser attachments (PN 11691). After removing the old rod, just attach to the hose and blast out the inside of the tank. Works very well. I find gunk from the old rod but never anything else - maybe because mine is always in use ?

Maybe drain yours between trips and flush at the same time. The only expense is the $6 hose attachment and some teflon tape.