batch water mixing???

[martym]

I was wondering if anyone mixes large batches of water for their changes and store it. I have a 10g nano which I do a 1 gallon change every week.I mixed 5 gallons of ro/di and crystal sea salt, have a heater and power head in it for circulation. After about a 2 weeks the water went brown. What did I do wrong? Mixing up only gallon of salt water per week is hard to get the ratio right. Any suggestions?