wrote

So far, I've only found Gunk anti-rust and water pump lube for car
radiators, $2 for 11 oz to protect 5 gallons or so, ie about $400 for
1000 gallons, and a KPR VCI powder that costs about $20/lb and
needs 0.25% concentration, ie about $400 for 1000 gal.

Nick, here's what I've been using in the heat storage system I built from your
suggestion of using old fuel oil tanks, some years ago. 2 parts to it, a rust
inhibitor, which costs about $15 a gallon and treats 300 gallons and an oxygen
scavenger, at about the same price per gallon. They have a chemical engineer
available, who may be able to answer any questions.

http://dwdavies.com/water_treatment/boilerkleen400.htm

and Stop-Rust #200 oxygen scavenger which is the very last item on the page.

http://dwdavies.com/water%5Ftreatment/

Hope you find this helpful.

Steve Young