Hello mr. Apparently, something the plant needs isn't being provided. This could be the wrong water chemistry, the plant may need a CO2 supplement or some other nutrient. Plants need nitrogen, phosphorous and potassium. They also need proper lighting. Low light plants don't need more nutrients than the fish provide and light that comes to 3 watts for every gallon of tank volume. More demanding plants will benefit from CO2 and lighting that's in excess of 4 to 5 watts for every gallon of tank volume. The thing to do, is to research the needs of the plants you keep to determine if you're giving them what they need.
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