Red Sea and IO are ok for fish tanks.
Try using Seachem Reef or Tropic marin reef (not PRO unless you dose and have a calcium ractor) if your tanks contain corals. They will really perk up.
Red Sea is harvested directly from the ocean and the salts are claimed from that sea water. OK great right?????? Weeelllllll............ Think about all those toxins that could be released into the water and then be directly concentrated and put into your tank. The story goes that RedSea contracted with a water purification/desalination station. There is a pipe about 1/2 mile out int othe ocean. The salts are removed in the first step. Redsea buckets those salts and ships them around the world as their salt mix.
IO industry standard. Been around forever and is cheap. However better and more specific mixes have been hitting the market and making improvements. Reef crystals is an IO product for reef tanks. Problem is they do not promote the product enough for people to realize that they actually do have a reef tank mix available.