When I'm ready to rinse the tank I use a home made wand similar to the Toilet / Black Tank Hose wand with a valve. The difference is I made it out of 1/2" copper pipe with a cap on the end. I purchased and assembled the following.
1- 1/2" Sweat Gate Valve
1- 1/2" Copper Male Adapter
1- 1/2" Copper Cap
1- 2ft length of 1/2" Copper Pipe
1- 1/2" MPT to Female Hose Adapter
I cut a 6" piece off the end of the pipe and soldered it between the 1/2" male adapter and the gate valve. Soldered the cap on the 18" piece of pipe and soldered the other end to the gate valve. Do check the flow arrow on the valve!. I then screwed the hose adapter on with teflon tape on the threads. I then took a battery drill with a 1/16" drill bit and drilled 3 holes staggered in the end of the cap, 2 more at a 45 degree angle through the solder joint where the cap meets the pipe. Then I just drilled 6 holes around the pipe back to 6" from the cap and staggered them. Hook your hose up and you'd be amazed the pressure of the water coming out. Watch out for those 45 degree holes because if done correctly they shoot back towards you. This wand will fit in any water heater tank drain. Just insert in the hole in the tank then turn it on. move it around, back and forth and the residue rinses right out.