Lots more info is needed before anyone can attempt even a wild guess at what the problem might be.
Close up pics of the damaged plants
At least one full tank shot
Exactly what you are dosing, how much, how often
Exactly what kind of light, distance from substrate
CO2 info, how it's diffused, how much, bps, etc
Filtration / circulation equipment
Any other known water parameters
Provide as much of this info as you can, then someone may be able to offer good advice.
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