I got a gravel vacuum specifically designed for tanks "up to five gallons"...though I'm sure siphoning with air line tubing would work, too! I got my thing at PetCo for...I think about $5 or so. That's what I use on my Betta's 2.5 gal tank... I don't really measure how much I take out, it's almost half. I do it once a week, though I also have a filter too, so that's why I slack off