My 10G lidless needs easily 1L a day in water. I get frustrated because unless I design an ATO system I'll never be able to leave my tank for vacation or anything of the sort.
BTW use RO or RO/DI water as often as you can, and exclusively if you can. I can't stress enough how important good water is, it is literally the life of your system. It will dictate your fish's health, partially your coral's development, and most certainly the overall success of your tank. Take the extra step to acquire the water, it will save you stress, high blood pressure, loss of hair and suppress the urge to go on a killing spree when your tank gets sick.
A good salt mix is key too. :D