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I doubt I will still have tanks in a couple of decades Suey.

Right I am now starting up my 650 liter marine. Got the salt and live rock coming this month. Then I can get my FOWLR cycling. I should have gone marine years ago.

You need to buy the equipment that is rated for your size of tank. Would you buy a filter that would only handle 1/4 the volume of your tank. For the Hagen to be any good you would need at least three (3) of them on your tank. 3 x 90 = 270/ your tank being 260, and that would cost you a bomb in refills as they would last at most 2 weeks if they are running 24 hours a day.

If you leave your Kh at 3 or 4 your pH will need to be around 6.8 to make the CO2 of any use. The more alkaline your water the less CO2 is present. pH, Kh and CO2 are all interconnected as CO2 in water creates a low pH and alkalinity reduces the amount of CO2 present.

If you start reducing your pH obviously your fish will start to suffer if the require a high pH. Before using diffused CO2 you need to research its effects and uses first. As has been said you shouldn't leave it running over night especially if you have plants in the tank. Adding CO2 lowers the pH of the water, making it acidic, and dissolves any calcium carbonate in your tank that in turn provides the aquarium with more calcium and alkalinity. So you need to be careful.

For your tank you need something like the JBL ProFlora U402 CO2 Set. The hagen is like trying to break a rock with a spoon it is not good enough. You can do it a lot cheaper than the JBL if you source, cheap CO2 extinguishers, regulators, solenoids, bubble counters off Amazon or Ebay.

Or use liquid supplements rather than diffused CO2. Something like Flourish Excel Co2 Plant Supplement.

If you do that your hair algae will get worse. You need to reduce the amount of light not increase it. Excess light is one of the causes of hair algae. You still need to increase the amount of CO2 to get your ph down to 7 and raise your kh to 10 to make the CO2 any use in the first place.

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That is a waste of Co2 by doing that as you are wasting 2 thirds as much CO2 as you actually use if your lights are only on for 8 hours a day. That means a 90 litre rated CO2 bottle will only last enough for 30 litres as 60 will be wasted.

Violet is completely correct. When the lights are out, the plants give off CO2 into the water that can double the amount of CO2 in the tank. This sudden rise in CO2 over night will drastically change pH levels for the period the lights are off and then change again when the lights come back on next day. This sudden change can cause illness in your fish and as violet said the sudden rise in CO2 can suffocate your fish by reducing the amount of usable oxygen in the water. Co2 should be used with great care.

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Hair algae can be caused by excessive levels of Phosphates and other nutrients as well not just low CO2 levels. Have you checked all nutrient levels?

To get Your CO2 to a level where it would do any good you need enough CO2 to get a pH of 7 and a KH of 8.