The light fixture you have is known to have a low end reflector/ballasts which means another company's exact same fixture will produce more PAR than your oddysea.
When i first started i opted for a cheap fixture and bought the 4x18 T5ho fixture for my 10 gallon tank. I run all 4 lights and find its perfect for this setup. However if i had got say a Catalina fixture i would have only needed a single light fixture for the same results. With regards to how much C02, just keep checking your fish, obviously if they are gasping for air then you have too much.
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