With very high light you are driving the plants to grow very fast. That means those plants that are ordinarily fast, aggressive growers will be that and more. There will be a shortage of CO2 available to the plants, because having non-limiting CO2 would kill all of the fish. So, the aggressive growers will quickly grab most of the CO2, starving the less aggressive growers. Those would become unhealthy plants, and algae would quickly colonize them, killing them.