70 micromols of PAR isn't extremely high light, so you can use that continuously if you want to treat the tank as a high light tank. That means you absolutely need to keep the water well oxygenated so you can jack up the CO2 concentration to match the plants carbon demands at the fast growth rate that high light causes. To get that CO2 to the substrate level for the hair grass you also have to make sure the grass is waving in the current, demonstrating that it gets "fresh" water continuously. And, you have to be very conscientious about doing tank and filter maintenance, to avoid causing algae with the buildup of organic debris. Then you need to adjust the CO2 bubble rate upwards just a little bit, every few days, while keeping watch on your fish, and observing the plants to see what effect the increases are having. Do this until you get good growth, and until the last increase didn't improve the growth. Finally, if you do start getting BBA, remove it, kill it, immediately or you will soon have a black carpet. If you enjoy all of that work, you will also enjoy the rewards of having a good high light tank.