i have no idea if this is safe or not but I wonder if you could add EDTA, hoping it would chelate the iron in the rust. it would do that to Ca too i think. but I couldn't tell if it works on iron oxide. i'm not a chemist just know the word chelate from gardening. where would you get EDTA raw IDK, it's a preservative in pickles but that probably has sodium salt in it so no don't add pickle juice. maybe there are already salts in the tap. citric acid might work too. that's probably in pH down. easier than looking for EDTA. EDTA might kill good bacteria too for all I know. so i just convinced myself. try adding pH down since no one complains about that here. see this. maybe helps
Citric Acid
Another crystalline organic acid that actively dissolves rust without attacking the metal is citric acid. It occurs natural in many fruits and has the added benefit of not being appreciably toxic. In fact the supersoar candies sold in stores are lace with crystals of citric acid! None the less, eating citric acid is not recommended as it might damage your tooth enamel. Like oxalic acid a fairly dilute solution can be used. When rusty parts are left overnight, the rust will be gone. Citric acid is superior to oxalic acid in that the iron salts formed are soluble where as the oxalates formed are not very soluble. Oxalic acid can leave a green deposit on steel that has been treated for rust. The citric acid forms iron citrate in solution which is reduced in direct sunlight to a colorless compound. For this reason clothing with rust stains can be treated with salt mixed with fresh squeezed lemon juice and left to dry in the sun. The rust stain will first turn green forming ferric citrate but then the UV rays from the sun cause the citric acid to reduce the iron to ferrous citrate which is white. There still is a crusty solid left behind so washing with the laundry is necessary. I also use
citric acid as a shower cleaner as well. It removes soap scum from shower doors and walls and clears hard water deposits that clog shower heads.
hopefully some expert chimes in
Edit: Molasses might do something with iron too.