Fwiw, CLR calcium/rust/lime remover does a good job on more stubborn hard water stains. It rinses clean with water, I've never had a problem using it where fish or shrimp are concerned. Rinse and rinse again is my motto no matter what I use.
If you do use vinegar, heating it up makes it work better and you can get vinegar for pickling that is stronger too, 7% rather than 5%. Costs a bit more, works better. In some places you can even get 10% vinegar. Be certain to never mix it with bleach, you'll get a foul gas that can burn your throat and lungs, even be fatal if you got too much of it.
Baking soda works best used as a paste, mix it with just enough water to make it pasty, scrub with it. Razor blade is best for really stuck on stuff, and you can often find that older tanks have a permanent stain at the water line, due to etching. No amount of cleaning or scrubbing will remove etching. The only cure is polishing, which is very, very time consuming and hardly worth the effort. The line will not show when it's wet in any case.