I am looking to replace an anode in a A O Smith 75 gallon gas water heater that is 26 years old. There is no odor associated with it and I did change out the anode 6 years ago which was almost gone. I did not make a note of the specs for this replacementbut will so so from now on. I have googled and googled but can't determine which material or materials is best for my situation- maganese, aluminum, aluminum zinc/tin mix or???

I live in Central Oregon and the water here is quite pure-I don't know its pH but will find out on Monday from the City. I also read in your postings that a 50 gallon hot water heater is almost twice as expensive as a 40 gallon HWH. If this is correct, then my 75 gallon heater must cost twice as much operate as a 50 gallon tank. (There are only 3 of us living at home and we take short showers). Are there stats on this type of comparison available. By the way, A. O. Smith reps were not able to give me any info on my existing heater it terms of efficiency ratings as they say it was never rated. When I asked them for just a ball-park figure so I might better be able to properly consider keeping my current HWH or buying a new AO Smith, they declined to do so so I am inclined to look for a Bradford. I was surprised at their lack of interest.

There is not much difference in price between a 40 & 50, but 75s are much more expensive now, assuming you can find one.

The heater originally came with a magnesium anode (here on the east coast, anyway-maybe anode selection is based on geography?). The difference here between gas 40 and 50 WHs is very minimal.