I recently built a tank using sand as the substrate. I finally perfected a way to change the water and not displace all the water when adding it back in. I did run into an issue though. I've been changing my water about twice a week, sometimes three times. Sometimes when I get bored I just decide to clean my tanks....wish I did that with my house instead but oh well. So I have a smaller vacuum that I use on the 10 gallon tank that I use first to get in the tighter spots and suck up the poop. Then, once all small poop chunks and tight spots are cleaned, I get out the big vac that I use on my 45 gallon and suck up the poop from the bigger spots. I move the decorations and clean under them. When this is done, I've usually gone thru 10 gallons of water. I pull out my vacuum and add in the new water. After my new water is added, I'll look into the tank and it seems like all the poop that I just vacuumed out is RIGHT BACK where it came from. What gives? Is there a vacuum specifically for sand tanks that will actually pull the poop out and keep it out? It just doesn't seem like the vacuuming is actually cleaning the tank, just dispelling the poop. Any suggestions on how to proceed with this?