Diatoms will out compete both algae and plants. However the sequence is that first other single celled algae are starved out and only later plants stop growing. The impact on plants depends on the dosage. Recommended Nualgi dose of 1ml per 1000 gallons per week ensures that only the algae is impacted and not the plants. If any impact is seen on the plants then the Nualgi dosage can be reduced.