Disclaimer: I know next to nothing about fish. That said: if you want to lower the pH, muriatic acid will certainly do so, and it won't introduce any ions to a salt water pool that aren't there already (it will increase chloride ion concentration, which may or may not matter; it will also increase hydronium ion concentration, but that's the whole point).
Yes; there is no problem with most normal pool chemicals, such as chlorine algaecide acid, muriatic acid, and pH controls.
You can try the following cleaning solutions; muriatic acid, bleach solution and battery solution.
Sodium Chloride (NaCl) is table salt. Hydrochloric Acid (HCl) is found in Muriatic Acid, which is used in swimming pool care.
Muriatic acid, or a dry acid is the best way to lower pH in a salt water pool. This will be an ongoing demand in a salt pool. The PH factor for Sodium Hypochlorite (the type of chlorine that the salt generator produces) is around 13, which is almost 2 times higher than where you want to keep it ( between 7.4-7.6) as long as your generator is producing chlorine you will have a great demand for acid to lower pH.
Salt does not stain you probably have calcium stains. This can be cleaned with a solution of muriatic or hydrochloric acid. try a test piece first and be careful for your eyes.
Add acid
No. A salt system uses plain table salt to generate chlorine - hypochlorous acid - and sodium hydroxide. This means that it will raise your pool's pH, which will result in you needing more of a pH-lowering chemical such as muriatic or dry acid. People who use a salt-water pool will notice that the water feels much softer than a traditional pool. There is, however, still chlorine in the water. A UV system is less common. It is used as a secondary sanitizer only, as it helps to kill some organisms that are resistant to chlorine. It does not affect your pool chemistry significantly, and can be used in conjunction with a salt system.
Muratic acid or Sulfaric acid
Add muriatic acid. You could take a water sample to a pool store, and they will tell you how much to add. Or add 1/4 gal each day until the pH drops to 7.4-7.5. 7.5 is ideal pH for salt water systems.
Muriatic Acid is a colorless aqueous solution and is the less pure (industrial) version of Hydrochloric Acid, with the formula HCl. It is also named spirits of salt or acidum salis, it has a “sharp, irritating odor”.
Muriatic Acid is really just an older name for Hydrochloric Acid. The name (muriatic acid) also gets used more commonly in industrial applications such as brick cleaning and pH adjustment in pools where people are used to hearing called that.
Lots of pool salt, some acid, a little sunscreen, and a good automatic cleaner. Plus a salt water chlorinator.