You really have a lot of light for a 55 gallon tank, especially one that doesn't yet have CO2. Remember, it can take a couple of weeks for BBA to get going when the conditions are right for it - low CO2, big drops in CO2 concentration, high light, etc.
If you have an external reactor, one of the PVC 2" diameter tube affairs, a lot of CO2 will collect at the top of that and keep on adding CO2 to the water flowing through, long after the CO2 is shut off. That is a good reason to shut off the CO2 an hour or so before the lights go off. If you have an inefficient diffuser or reactor, and it takes 2 or 3 hours for it to build up to full concentration of CO2 in the tank, it is a good idea to start the CO2 an hour or so before the lights come on. But, if your CO2 diffusion method is a good one, or if you are using the CO2 mist method, there is no reason to turn on the CO2 before the lights come on, and you might as well turn it off with the lights too.
Get a drop checker to help you set the "right" CO2 bubble rate. That will make it a whole lot easier. The regulator should have two gages, one measures the CO2 pressure in the CO2 bottle, and it should show about 700-800 psi continuously, until the liquid CO2 is used up, then the pressure slowly drops to zero. The other gage shows what the regulated pressure is, and most people use 10-20psi or a bit higher for that, depending on how much pressure it takes to get the bubble rate they need. That is the pressure you adjust the regulator to produce. Then the needle valve is adjusted to set the bubble rate.
If you have an external reactor, one of the PVC 2" diameter tube affairs, a lot of CO2 will collect at the top of that and keep on adding CO2 to the water flowing through, long after the CO2 is shut off. That is a good reason to shut off the CO2 an hour or so before the lights go off. If you have an inefficient diffuser or reactor, and it takes 2 or 3 hours for it to build up to full concentration of CO2 in the tank, it is a good idea to start the CO2 an hour or so before the lights come on. But, if your CO2 diffusion method is a good one, or if you are using the CO2 mist method, there is no reason to turn on the CO2 before the lights come on, and you might as well turn it off with the lights too.
Get a drop checker to help you set the "right" CO2 bubble rate. That will make it a whole lot easier. The regulator should have two gages, one measures the CO2 pressure in the CO2 bottle, and it should show about 700-800 psi continuously, until the liquid CO2 is used up, then the pressure slowly drops to zero. The other gage shows what the regulated pressure is, and most people use 10-20psi or a bit higher for that, depending on how much pressure it takes to get the bubble rate they need. That is the pressure you adjust the regulator to produce. Then the needle valve is adjusted to set the bubble rate.
