CO2
I have a client that called me out to check his tank he'd been doing EI with for a long time. We looked at the dosing log, nothing out of the ordinary.....
The tank was perfect the last few months.
I looked under the tank.........the pump driving the CO2 reactors was unplugged. Apparently the last week's water change had the CO2 system unplugged for cleanign but forgot to switch it back "on".
Opps!
The tank looked pretty good for being a high light packed tank without any added CO2 for a week. Excel is dosed 2x a week, so that likely helped.
But.........back to your issue, the Giant Hygro all had holes on some of the leaves.
I've seen this consistently and many folks think it's K+, however, K+ is added in the form of GH booster and KNO3, to at least 6x what would be required vs N.
So I'd have to use up all the N @ 6x the rate relative K+ before K+ would be an issue............that's not happening.
Stunted tips, browning, rotting etc, species express their dislike for non stable or sudden low CO2 many ways.
I know this plant and several others very well in terms of deficiencies.
Algae, and plants are better test kits once you know and have confirmed the cause.
Regards,
Tom Barr