Yes the light could be coming from any source. I have a screen for my tank so when wife opens curtains it blocks the light from the window as it all adds up.( Dark room best to keep tanks in then only one light source). As Matty said though , Dirty tank, lack of GOOD maintenance, too much light, overfeeding fish and POOR CO2 implementation. Adding LCO2 will help but will also take some time to clear the staghorn like Months with regular maintenance, when was last time you cleaned the filter? I do mine weekly