Ok, so I am assuming you have an established tank. When you do water changes you syphon up to 30% of the water from your tank. If you use a gravel cleaner (which is just a ridged plastic tube of larger diameter than the syphon tube) it will suck up the sand along with the poo etc. Now, with sand you have a couple of options to prevent sucking all the sand out. I have black quartz and it is very prone to being sucked up.. To start with I do a sort of fiqure of 8 about 25mm off the bottom which disturbs the sedement and up it goes. Then after that I gently poke the tube into the sand and of course the sand fills the tube. Just before it reaches the top I put my finger over the other end of the pipe to stop the flow. The sand then starts to flow back allowing you to quite accurately replace it where you want....ie over exposed plant roots etc. Releasing your finger allows the flow to continue and repeat the proceedure. If you really need to replace all the substrate (why?) I do not see a way to do it without freaking out you fish so it would be best to remove them first. Hope this sort of helps
