I would say potassium or it could be high level of nitrates but without testing trace and all other elements you can't be sure. So if your nitrate are ok then add more ferts but go up slowly so you dont overdose and make algae farm.
That’s normal in java fern. They’re probably old leaves, they’ll produce baby ferns so when you see small white string on them, leave it. It’ll grow and fall off on its own when it’s big enough.
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