K is not typically a problematic nutrient. I have had 20,30,40,60,80 and 100+ K ppm levels in my tanks with no issues.
While he didn't notice ant issues, that doesn't mean the fish were not affected. excess levels of anything can have health impacts that may effect the quality of life for the fish without killing it. it might weaken the fish immune systems and damage internal orgins. Furthermore the primary reason for large water changes in an EI tank are to excess nutrient (potassium, sulfur, chloride, and many others) from building to dangerous levels (whatever those levels are. However if we could balance the supply of nutrients in the fertilizer to the plants needs much of this excess could be avoided perhaps fewer or smaller water changes would be needed and the fish might be healthier.
I agree with trying to balance nutrients with plant uptake but there are issues that are hard to resolve:
1. Not all plants consume nutrients at the same level. For example C4 plants need more sodium than the more common C3 plants. Some plants need more sulfur while others need less. There is no one magic ratio of nitrogen to potassium.
2. Depending on the plant and fish load and amount of food consumed will very from tank to tank meaning the nutrient needs of each tank could be different.
3. Tape water series in its composition. Some tap water may have high potassium levels but low nitrogen content. With fertilizer we add nitrogen to correct for that but in the process we push the potassium content even higher.
4. Most people don't know what there potassium and sulfate levels are or what the levels are for the micronutrients are. Potassium and sulfate test kits are hard to find. test kits for micronutrients are generally not available or very hard to find in most stores.
With all those unknown the best way to know what the ratio for your tank is. assuming you can find a test kit. the best solution Is to monitor the nutrient uptake by the plants and adjust the dosing level to try and compensate. However doing this can have unwanted side effects. If you decide to reduce potassium by reducing potassium phosphate dosage you could trigger hard green spot algae on the glass. Hard Green spot algae loves water low in phosphate. This can be addressed by scrapping it off the glass (more maintenance) or by increasing phosphate levels or by switching to a different phosphate fertilizer that doesn't contain potassium.
One could use calcium phosphate which unfortunately increase calcium levels and water hardness. Or one could drop potassium nitrate and try to replace the nitrogen with urea, ammonia nitrate, or calcium nitrate. Each of these has some limitations. Urea can breakdown into ammonia (which is toxic) Ammonia nitrate is also a explosive ingredient and may not be available in some places due to legal restrictions. Calcium nitrate again will increase GH.
There are limits to what we can do. It is probably very difficult to reduce potassium levels as much as you want.