Saltwater natives need distilled water?

Nothing wrong with my TDS meter. In 2012, our water provider's annual quality report showed their TDS readings were 352-413ppm! We're just towards the upper end of the bell curve.

... also sodium, potassium, sulfate, nitrate, iron, and other ions that have little or no relation to hardness. There's no such thing as a hardnesss meter, far as I know. Hardness is measured by chemical titration. Public water systems that use well water often have TDS above 300 mg/L, and if you have a private well it may be above 500.

Typically TDS is measuring calcium, magnesium, chlorides and silica primarily. Federal drinking water standards are for no more than 500 ppm for TDS.

Primarily can be an important word.

You can use distilled water in the aquarium in a number of situations: First, when the water in the tank evaporates, the minerals will stay. You can increase the amount of water by replacing the evaporated water with distilled water. You can also use distilled water to lower the pH in the tank. This also reduces the carbon hardness of the water. Additionally, when the distilled water is remineralized, you can use distilled water to replace all the water in the aquarium.

You can use distilled water in the aquarium in a number of situations: First, when the water in the tank evaporates, the minerals will stay. You can increase the amount of water by replacing the evaporated water with distilled water. You can also use distilled water to lower the pH in the tank. This also reduces the carbon hardness of the water. Additionally, when the distilled water is remineralized, you can use distilled water to replace all the water in the aquarium.

 Mineralized distilled water is also the best type of water to use in a fish tank, in my opinion. As for spring or purified water, I suggest adapting them for the needs of your fish. It may take a while, though, considering that not all spring water and purified water are the same.