Without aquarium plant fertiliser and really good light, there is no point adding carbon dioxide (CO2) because the plants need the nutrients and light just as much as they need CO2. If you don't have them in the right amounts, the plants don't do that well and algae will often thrive.
You are better off using an aquarium plant fertiliser for a few months and see how the plants do. Then add CO2 after if you want to. However, there is plenty of CO2 in the aquarium. It is produced continuously by filter bacteria and fish. You also get CO2 in the water from the air.
If you have a plant only tank without fish, or only a few small fish, then supplemental CO2 can help the plants. But in an average aquarium that contains a number of fish, you do not need to add CO2.
-----------------
KH (Carbonate Hardness)
Adding CO2 to an aquarium can drop the pH very quickly. This is due to CO2 being an acidic gas (the pH of CO2 is below 7.0). You need to make sure the aquarium water has a KH (carbonate hardness) over 60ppm (and preferably over 100ppm). The KH neutralises acids in the water and stops the pH dropping. If you don't have enough KH in the water, the pH can drop literally overnight and the fish can die.
-----------------
CO2 on at night
A lot of people turn the CO2 off at night because plants don't use it when it's dark or there is insufficient light. If you leave the CO2 running at night, you should have an air pump come on when the tank lights go off. The air pump will blow bubbles into the tank and keep the oxygen levels high during the night so the fish don't suffocate (die from carbon dioxide poisoning). In the morning about 15-30 minutes before the lights come on, you turn the air pump off and the CO2 levels start to build up. When the lights come on, the plants can use the CO2 and fertiliser.
-----------------
You need to monitor the pH and KH, and if possible the CO2 and oxygen (O2) levels in the water for a month or so before adding fish. This will let you fine tune the CO2 and make sure there is enough oxygen in the water for the fish, and that the pH doesn't change during the day or night. When everything is settled, than add a few cheap fish and see how they go.
-----------------
If you only have a few small plants in the tank, don't add CO2 until they have grown. They simply won't use the extra CO2 that you add.
