I heat filtered water (charcoal cartridge type) on my indoor natural gas stove in a large covered stainless pot to boiling, carry the pot to the outdoor stainless tank and reheat to over 180F. That answers your question, and it should be on the instructions accompanying the phosphoric acid. I fill the tank to about 1/4 inch from the top.
Some additional thoughts: the filter removes particles from the water; the boiling lowers the chlorine level; indoor natural gas is cheaper than propane; the wind outdoors sometimes blows out the flame; I keep the cover on the tank to keep the temperature as high as possible, even while cooking parts; have an old pot holder handy; start with large parts that should have a light, natural finish, then do hardware or items that need to be black; light finish cooking time is about 1 1/2 to 2 minutes, hardware is 8 to 10 minutes; I use Eastwood metal black to get hardware dark; add water as needed to keep the level up, water boils off.
One more thing, I have a five gallon plastic bucket that I use to pour the remaining used phosphoric bath into when it's cool, then add a cup or two of baking soda (Borax) to neutralize the liquid. After the liquid evaporates (that may take some time), break up the residual mess, bag it and dispose of. To placate the skeptics, add more baking soda, acid remains are a no-no, alkali is better.
Jim