I changed my large gravel nuggets with small gravel a week ago, and have had good results by vacuuming the gravel with about 10 gallons of my 29 gallon tank., taking out the old rock and putting in the new w/o ever removing my fish. Just in case, I left a cut-open milk jug full of my old gravel in case it would be necessary to keep it there to filter the water until the new gravel can get the beneficial bacteria. I still have it there but I plan on taking it out this saturday. My fish have been fine since the day after w/ no losses. I guess my strategy was to keep as much water the same as I could and also not bother the filter. I have been known to clean thoroughly it but not this time.