@Cobrac -- Thanks for the comments. Consistent with your suggestion, I have been constructing my own water for mead fermentation for a few years. My approach follows whatever authoritative books or scientific articles I could find that directly and credibly address the question: What minerals are required to keep yeast happy? And to keep beer/mead drinkers happy? Many of these sources are listed above.
The minerals serve two basic purposes: (1) they provide nutrients, and (2) they buffer / modify pH. And these minerals must be added within certain constraints. One constraint is that the minerals should not interfere with healthy yeast metabolism. For example, sodium competes with hydrogen for transport out of the yeast cell, so too much sodium makes it difficult for the yeast cell to maintain an optimal internal pH. And of course, the minerals are added with boundary conditions to ensure that they not adversely impact taste. For example, abundant chloride might be OK per se combined with normal Ca, Na, and/or K could create a salty taste. Similarly, potassium per se seems ok in large amounts but can produce a salty flavor in the presence of excess chlorine. Similarly, ample zinc can encourage yeast metabolism but then can create a metallic taste. And it's not just taste. Ample magnesium is good for yeast metabolism but too much can have a laxative effect.
Bottom line: Fermenters ignore minerals at their peril. Distilled or RO water has no minerals, so if such water is used then supplements seem necessary. On the other hand, tap water or even purchased spring water will have minerals but maybe the wrong ones or the wrong amounts. As noted, my own well water has too much sodium and too much chloride to be optimal.
Also, while honey itself has some minerals, it does not have enough to ensure a healthy fermentation.
FWIW, I'd be very happy to discuss this topic. But I don't think I can do it here. My simple question in the OP, which asked whether there are reasons not to add potassium, elicited a close-minded response. Like me, you view water management as a potential "next level" in mead making. I was told categorically that adding minerals is passé -- everything you might ever need is already included in Fermaid O. But I want to know what I am doing and why.
I think to approach this topic the right way I believe one has separate this into sensoric and yeast needs. They don’t correlate is my knowledge and experience. But our sensory perception is far harder to keep happy.
yeast doesn’t require much minerals and salts at all. And just to keep happy yeast won’t give you very much taste or profiled meads. However even if yeast don’t “demand” much minerals they can still added up to quite high levels without going to stall. Water all around the globe is very much different and all places get fermentation going and with different outcomes. So it is maybe time to explore different outcomes with different water. To manipulate meads properly we also have to take account for other factors as well. But by studying water properties maybe it’s possible to get some better understanding for certain outcomes of flavor. Basic thing is to use our own sense, does the water taste well in both room temp and cold. It’s probably a good water for beverages , but to get that extra fine tuning/tweaking maybe it’s a good idea to study the properties of the water. What have you got to know ow so far of different salts and minerals? Any specific tryouts and side-by-side tests?