Are we talking about a freshwater tank or a saltwater tank? I don't generally recommend distilled water for freshwater tanks. I have not done saltwater, so I am not going to give you any advice there. For freshwater, all you generally need to is buy a dechlorinater like Prime, Amquel, Nutrafin, or anything like that and add the amount suggested on the bottle. No need to boil or do distilled. That is way more hassle than is necessary.