Bleach is the thing to use. Working in the yard with the hose is best, weather permitting. I've done 10's & 20's in the laundry room, & had to mop up afterwards.
I use a couple of cups in a 5 gallon bucket of water, dump it in the tank that needs cleaning., scrub it good & rinse. When I fill it, I fill & use double the dechlor, just in case there is any bleach residue left. Drain again, & fill with water & dechlorinate. This is one of the few times I run carbon for a few days. Any impurities that may have been left from the supposed "pure" bleach will be removed by the carbon.
Many people advocate using salt water to clean tanks & equipment. This will kill most bacteria, but not viruses. I use bleach on gravel vacs, draining hoses, nets, most anything that needs to be cleaned in the fishroom. I just rinse them with tap water after the bleach solution, use them right away in many cases.
My mom worked for many years in the lab & blood bank of a level I trauma center here in Chicago. Lots of blood from questionable sources. They always used a bleach solution to clean up. I think if mom trusted her health & well being to bleach it can be trusted to clean diseases out of a used aquarium.
Tolak