If he's adding KNO3, it's not likely he needs more K+.
Sounds more like CO2 related.
It's not just java, it's Anubias as well.
Non CO2 tanks easily can have very different CO2 levels, one larger tank might have more, via fish, less water movement etc, or organics built up, the other much less.
The light might seem pretty close, but without a light meter, and some experience with those, you really cannot say.
Seems much more like a light/CO2 related issue, and I have a ton of these plants. I do not add extra K+.
The other issue is the biomass of plants added relative to tank size.
Adding a small plant to the large tank will not have the same impact as adding a large plant to a small tank... and that assumes that the CO2, nutrients are the same.
There's just a lot more plant that has less loading of nutrients in the smaller tank. Many of the holes folks see often are CO2, the plant cannibalizes itself for carbon, K+ can do this as well, but that does not mean K+ is the only cause for it, there may be more.
I've ruled out K+ in Java fern that turns black and gets holes in CO2 enriched tanks as well as one non CO2 tank.
Had plenty of K+, the issue persisted.
Regards,
Tom Barr