Pics of the Hygro and other plants would help, and a full tank shot
And more info about your substrate, light, and CO2?
Also could you list the actual ppms youre dosing? Listing the parts of everything (or mls or tsps) doesnt really say much.
Are you dosing any micro nutrients or Fe?
Here is a nutrient calculator that'll figure the ppms for you. https://rotalabutterfly.com/nutrient-calculator.php
"Premixed" will give you brand name products like Flourish liquids, DIY is for dry salts like KNO3 etc.
Hygros have a reputation for being K hogs. This is primarily because most varieties are quick to display the classic pinhole symptoms in lower leaves.
This can sometimes be a straight K deficiency, but can also be an "induced" deficiency brought on by some other nutrient imbalance.
Its sounds like you probably have enough K, but again its hard to say without knowing the specific ppms.
Do you know the total GH of your tap water, or whatever kind of water you're using? Specifically how much Ca and Mg is in thereto begin with? That'll help knowing if you might be low on either one of those.