Ava's got it right...start on one side and work your way over to the other side...if you only want to do a gradual change. This gives the bacteria a fighting chance keeping a foothold. But don't do a filter change until you're completely done and don't do a filter change just prior to the change either. You want to keep as much of your good bacteria as possible in your tank. Easy way to add sand to a tank filled with water is...wash the sand and scoop the wet sand into a small container and lower into tank and slowly pour out. Or another method is to take a 2liter plastic bottle...cut the bottom out, place wet sand in it. Place finger over top or neck of bottle( the end you drink out) lower into tank, remove your finger and sand will slide out the neck or you may have to shake bottle a little. Test your ammonia and nitrite levels because you will get spikes every time you take out substrate or add new. Just move your rocks, decorations, ect.....around keeping them in the tank. So the bacteria doesn't die on them. Check levels...
Another way is to plan ahead. Do a ammonia and nitrite level check, to make sure you don't have a level problem already. Top off tank couple days before. Don't feed fish the day before. Have sand washed and ready. Fill enough buckets with water from the tank to put decorations, plants, fish, ect in buckets. Turn filter off, if you haven't already from the lower water level. Put decorations, rocks into buckets... then the fish. Take old gravel out. Vacuum bottom of tank. Place new substrate in. Add enough treated water to tank to be able to turn filter back on. Add rocks, decorations...set them up as you want. Then add fish. top tank off. Add stress coat if you want. Leave lights off rest of day. Check ammonia and nitrite levels. Next day check levels. Turn lights on, feed fish as usual. Check levels...be ready to do water changes...it's like a new tank set up...you will get a spike.
Good Luck
