because table salt is 99.99 percent sodium chloride.
the composition of salt water is much more complex and very important to sea life and fish that live in brackish water.
Element ppm
Chlorine, Cl 19,500
Sodium, Na 10,770
Magnesium, Mg 1,290
Sulphur, S 905
Calcium, Ca 412
Potassium, K 380
Bromine, Br 67
Carbon, C 28
Nitrogen, N 11.5
Strontium, Sr 8.0
Oxygen, O 6
Boron, B 4.4
Silicon, Si 2
Fluorine, F 1.3
Argon, Ar 0.43
Lithium, Li 0.18
Rubidium, Rb 0.12
Phosphorus, P 0.06
Iodine, I 0.06
Barium, Ba 0.02
Molybdenium, Mo 0.01
Arsenic, As 0.0037
Uranium, U 0.0032
Vanadium, V 0.0025
Titanium, Ti 0.001
Zinc, Zn 0.0005
Nickel, Ni 0.00048
Aluminium, Al 0.0004
Cesium, Cs 0.0004
Chromium, Cr 0.0003
Antimony, Sb 0.00024
Krypton, Kr 0.0002
Selenium, Se 0.0002
Neon, Ne 0.00012
Manganese, Mn 0.0001
Cadmium, Cd 0.0001
Copper, Cu 0.0001
Tungsten, W 0.0001
Iron, Fe 0.000055
Xenon, Xe 0.00005
Zirconium, Zr 0.00003
Bismuth, Bi 0.00002
Niobium, Nb 0.00001
Thallium, Tl 0.00001
Thorium, Th 0.00001
Hafnium, Hf 7 x 10-6
Helium, He 6.8 x 10-6
Beryllium, Be 5.6 x 10-6
Germanium, Ge 5 x 10-6
Gold, Au 4 x 10-6
Rhenium, Re 4 x 10-6
Cobalt, Co 3 x 10-6
Lanthanum, La 3 x 10-6
Neodymium, Nd 3 x 10-6
Lead, Pb 2 x 10-6
Silver, Ag 2 x 10-6
Tantalum, Ta 2 x 10-6
Gallium, Ga 2 x 10-6
Yttrium, Y 1.3 x 10-6
Mercury, Hg 1 x 10-6
Cerium, Ce 1 x 10-6
Dysprosium, Dy 9 x 10-7
Erbium, Er 8 x 10-7
Ytterbium, Yb 8 x 10-7
Gadolinium, Gd 7 x 10-7
Praseodymium, Pr 6 x 10-7
Scandium, Sc 6 x 10-7
Tin, Sn 6 x 10-7
Holmium, Ho 2 x 10-7
Lutetium, Lu 2 x 10-7
Thulium, Tm 2 x 10-7
Indium, In 1 x 10-7
Trebium, Tb 1 x 10-7
Palladium, Pd 5 x 10-8
Samarium, Sm 5 x 10-8
Tellurium, Te 1 x 10-8
Europium, Eu 1 x 10-8
Radium, Ra 7 x 10-11
Protactinium, Pa 5 x 10-11
Radon, Rn 6 x 10-16
as for killing the ich parasite with sea salt, it has plenty of sodium and chloride to do the job.
