get the ntuc brand, selling 3X1.5Lt for $1.35... dont change only when your tank start to turn cloudy, cos by that time your LS all mati liao... i would suggest to change water every 1 week, to play safe for a nano. water get pollute easily.
maybe like wat u mention just now in your thread... get your mum to say yes n work for her next month lor... haha... anyway i'm getting a seio 820 this sat from one of the reefer also... must have more flow for my tank... haiz, alot more to do with my tank... must make it look nicer