I change 50% of my tank's water every sunday, and do an light gravel vacuum. I just roam around with my siphon, and only lightly disturb the substrae if I see poops or food in the siphons vicinity. I think how much you vacuum and how muchwater you change depends on how many fish you have in the tank, and what kinds. for example, I probably wouldn't do as many water changes if I didn't have my BN pleco in there, because she makes a lot of waste and mess, because plecos come from faast-flowing rivers, so they're mess and waste is immedately swept away, and they don't worry about being messy. and when I had only my betta, I did a 50% only once every 2 weeks, because they make a lot less watse and mess, an adaptation to living in small, filthy, ponds, no doubt.