Do you hvae any fish in the tank? What is the SG
When you top off your tank every day or so, you should always be adding freshwater, as the salt in the water won't evaporate. Adding saltwater would continually increase your salinity.
I do monthly water changes of about 20-30% in my SW
tank. Algae could also come from a number of sources. Too much light, high phosphates or nitrates.
Also, what color algae is it? Are you still cycling your tank? If it's diatoms (brown algae) and you have nothing in your tank yet, just let nature run it's course and leave the algae there. It will go away as soon as your cycle catches up, and they are quite common in all new tanks.
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