Also - my 2cents - don't waste your money on a hydrometer - they're inacurate, cheap and flimsy (I went through 3 before I gave up and bought a portable refractometer).
Here's what I do when I do a water change: (this I gleaned from the board and have modified it a bit):
Top off your tank with fresh RO/DI water before you do anything else.
Turn off all your water circulation.
Use a small brush to scrape your rocks to dislodge any algae. Scoop up any that rises to the surface with a net.
THEN, use a small airhose to siphon out grunge, gunk, filaments and other detritus that accumulates on rocks. Don't get near your substrate with this hose or it will clog.
Then use a bigger diameter hose to finish sucking the rest of the water - if you have algae accumulating on your sand bed - suck it out using this hose (don't worry about losing your sand - you'll get it back later on).
After you've finished filling the waste water bucket, get your algae pad out and clean the glass. Start at the bottom of the glass (by the substrate) and firmly draw the pad up to the water surface - then rinse it out in the waste water bucket. Repeat until you've completed cleaning the glass (always using the waste water to rinse out and clean your pad).
Dump the water - but before you dump the very bottom of the waste water bucket , grab a tight-weave strainer, dumping the rest of the water through the strainer to catch your sand. Then re-deposit the newly cleaned sand in the tank.
DON'T FORGET TO TURN YOUR PUMPS BACK ON!!!!