Tap water is the main cause of most problems growers get. It's also the main cause of pH problems. pHing your feed water to 6 or so does almost nothing to the pH in the root zone when there's a buildup of alkaline minerals like the carbonates abundant in tap water. I can get all the free tap water I want from friends that live in town but I wouldn't drink it and sure as hell wouldn't try growing my plants with it. ProMix, hydro nutes, (ph Perfect) and RO water all the way!
Every time you water with hard water the minerals get left behind in the pots just like they build up in your kettles and coffee makers. Depending how hard your water is you may need to flush as often as every few waterings to keep toxic salts buildup at bay. Town water here is over 400ppm, very high in sodium and lots of other things. I called the town and they emailed me a water analysis report for free so anyone using tap water should do the same. Invest $100 in a cheap RO unit. One of those 5gal/day units is good enough for the small grower if you have something to store water in like a plastic rain barrel. Stick a cheap airstone in it and you'll have perfect water for your high value crops.
If you are adding CalMag to most tap water . . . don't! Hard water has more Ca and Mg in it than the plants can use already so adding more just makes it worse. If you start using RO water then a little CalMag is need tho most quality nutes have enough for most types of plants. Pot does better with higher levels but most people use too much and then lockouts happen. It's not a magic bullet folks. Just another tool in the grow shed. Even with coco if you're using hard water you likely won't need to add CalMag. What most people call Ca deficiency is actually toxic salts buildup. Older leaves are the last to be affected by low Ca as it's an immobile nutrient and once it's in a leaf it stays there. Lack of Ca starts with very dark green new growth, weak stems and very slow growth then new growth discoloring with yellow to purple hues then new growth coming out distorted. Mg on the other hand shows up first in older leaves like low N with yellowing starting there and progressing up the plant with new growth being the last affected because like N it's a mobile nutrient and the new growth will take it up from the older leaves.
Mobile nutrients are NPK, Mg and Zinc and they move around the plant when needed elsewhere so older leaves are affected first. Everything else is immobile and if there's a lack new growth shows it first.