WRS, please don't do a 100% H2O change. If you are going to remove your GRAVEl and your WATER you may cause your tank to recycle. I recently converted all of my tanks to sand when I was having a major problem regulating my H2O parameters (due to a power outtage my tanks recycled cause of bacteria loss).
Anyway, when I converted from gravel to sand I siphoned out 75% of my water. Half of that I dumped and the other half I put in a separate container (I did this for four tanks separately). I thne removed all of my ornaments and plants and large rocks. I kept my fish in the tank during all of this. Some people recommend removing them, others recommend leaving them in. It was easier for me and less stressful for the fish if I just worked around them rather than chasing them with a net and dumping them in a holder tank and then repeating that process.
Once I had the large stuff out I scooped out the gravel with a large net, this way I got just the gravela nd no water. I kept my airstones and filter running throughout all this, but put a piece of filter floss on the filter cartridge to catch the extra detritus. OH YEAH - IMPORTANT- UNPLUG YOUR HEATER BEFORE DOING THIS> OR ANYTIME YOU REMOVE A LOARGE PORTION OF WATER.
Sorry for shoutin but it is important to do that.
Okay, well, once the gravel was out the water was, well, nearly MUD. Then I SLOWLY added the sand cup by cup. Add each cup as low to the bottom of the tank as possible, if you just plop it in it will swirl about and most of it will float and contribute to the water pollution. There was so much cr*p floating around I wondered if my fish were going to make it and if I had made a mistake in undertaking this process. After I got about two inches of sand in I added back all of my ornaments and stuff and then filled the tank with the reserved water that i had removed and the rest with fresh RO water. Needless to say my water cleared up after about an hour or so. My fish seemed okay and I kept the lights on.
You can add Kent PolyOx to the water after this to help clear up the water faster. Also, I did not feed my fish for about two days afterward. Once everything had settled/filtered i vacuumed out the rest off the surface of the sand and removed the filter floss and cleaned the filter cartridge. I did not replace the cartridge, just swished it around in some clean RO water to get the extra gunk out.
Hope this helps, hopefully you will get this in time before chaging 100% of your water. If you want you can PM me for more info.
Good Luck!
p.s. i have had crystal clear water ever sine and my H2O parameters are absolutely perfect. And sorry for the typos.