For the amount they need the only mineral that could potentially be an issue is iron and that is if your horse is already getting excessive amounts. The problem comes when they are allowed to consume salt at will AND they eat more than they need in an amount(within reason) that is not offset by water consumption. Depending on the work load and amount of sweating they could need from 2 tablespoons to 4 tablespoons a day - that is roughly a minimum of two pounds a MONTH. If you are putting out a 50 pound block and they are eating it by the chunk then they are likely getting more than they should have BUT if it is not extreme and there is fresh water available at all times then it usually isn't a problem. For one horse with weather exposure that block should last at least a year and a half. If it doesn't last 3 months then you have a problem. That says you need to be adding it loose to the feed. Most horses will self regulate some won't.
Water consumption is based on whether or not it is available, clean and the correct temperature. the more salt they eat - the more water they need. That means if it is not available free choice then you (G) need to ensure that an appropriate amount at a temperature they will drink is available. There are places you will read that they need 5 to 10 gallons a day. Mine drink much more than that - especially in the summer. If it is too hot they won't drink, if it is too cold they won't drink.
All of that said you feed what your horse likes and eats best. Some really like the taste of sea salt or mined salts like redmond or himalayan. When you go to any salt that has the potential of being contaminated in its natural state then you want FOOD GRADE. I'd trust the redmond salt over the himalayan for aninmal use but that is paranoia on my part with unscrupulous manufacturers.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.