i have a 55 gallon and I top off about 5 gallons of water once or twice a week and do a 5 gallon water change about once or twice a month.
so for fresh water your spending about .50 a week or about $2 a month. add another buck in there for sw for changes and you're lookin at an average of $3 a month for water. in a year you'll spend about $36. Thats really not too bad... until you consider the "uh ohs" that come up. Something has gone haywire and your nitrites are spiking. you need to do a water change
ASAP, but it's friday night and the
LFS isn't open this weekend cause this is the fourth of july weekend. You see where I'm going? And if your like me... the
LFS isnt just down the street... it's a 20+ minute drive. Your buying piece of mind and convience. I like the fact that I can have a 30 gallon rubbermaid full of fresh
RO water on hand at all times.
JMO, if the
LFS is close and you have the means to get extra water from them to keep on hand in case of an emergency, then maybe you don't need one. Up to you