As far as just the plants are concerned, you can use 100 ppm of CO2 and they will be happy. But, the fish will be dead. The limit is based more on what the fish can live with than what the plants need. The more dissolved oxygen you have in the water, the more CO2 the fish can live with. The more light you have, the more CO2 the plants need to grow as fast as the light is forcing them to. With low enough light, the plants can grow with just what CO2 dissolves into the water from the atmosphere, but more CO2 will make the plants grow faster and more healthy.