Due to the sticky nature of the sodium molecule SW bonds and holds onto other elements far easier than fw. Tapwater issues can range from low to high ph levels, high levels of heavy metals, dissolved organics leading to high nitrate levels, Phosphate issues, limewater issues, as well as toxin build up from terrestrial weed control, and silicates.
Generally the issues caused by the above are diatoms from silicates, bacteria such as red slime from dissolved solids, fuel for unwanted algae from solids and po4, fish illness from uncontrollable nitrates, shell fish death from heavy metals, coral death from nitrate and po4, stress from too much calcium, and only in extreme cases usually from well water near farms, PH instability and toxin build up.
All municipalities water supplies are different in content so not all issues are always relevant - my opinion better safe than sorry.