The yellow leaf on frogbits look like nitrogen deficiency, or lack of light. Perhaps your frogbit already had the yellow leaves before you put in your KNO3. The black patch is the first time I'm seeing this. Looks like oil contamination. You have something above that is dripping oil?
Your CO2 is low, which is probably why plants are slow to grow. Dark markings on frogbits are normal. But sudden crypt melt points to sudden change in water condition because my crypts melt even when I move some frogbits away, as there is suddenly more light available than they're used to.
Is this tank an established tank? How old? add more CO2 and maintain your fert. Perhaps you can add some KH2PO4 and forget about the MgSO4. Try this out for a week and see if conditions improve.
Warm regards,
Lawrence Lee
brothers, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable--if anything is excellent or praiseworthy--think about such things.
Philippians 4:8
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