FindingMe, I don't know anything about citric acid, but we have really hard water too and the only thing that works is baking soda, borax, and vinegar. You mentioned the baking soda, so I was wondering if you've tried all 3 together? I sprinkle the baking soda and borax in the tub and then put vinegar on my sponge an it works great. I've tried everything and that's the only thing I've found that works. Hope it works for you. You at least won't have to worry about the fumes. 
Okay. I've tried the baking soda, borax, and vinegar mixture. It got some of the buildup off, but NOTHING is removing the buildup right around the jets. I've tried scrubbing is with a stiff bristle brush, an old toothbrush, a scrubby sponge, along with the borax/baking soda/vinegar mixture. The stuff just won't come off. I had great success with my kitchen faucet, though - mostly just soaked it in vinegar for hours, and the majority of the build-up just came off easily. Way cool.

But my faucet is metal, and my tub is fiberglass, I think. I've noticed that the build-up doesn't seem to come off very well when it's coated anything more "plastic". Does anyone have any great tips for cleaning calcium deposits off of fiberglass or plastic?
Also, I haven't tried using Coke, but from what I've read, you have to let it sit on for hours. I haven't figured out a way to soak the outside edges of the jets on the sides of the tub with Coke... I can just pour it over them, but I doubt that's going to help much. I tried spraying them with vinegar, but it either didn't sit on there long enough, or it just didn't help.
Any ideas, or am I gonna just have to start saving for a new tub? (Or more likely, learn to live with the calcium deposits!

)