What i would do is a bit different than
Lalo J.
remove all the decoration - buy a pair of 5 gallon pails and make sure they are very clean (approx $3.5 each at many retailers). Put about 3 to 4 gallons of water in the tank and put the fishes in them. Then drain the rest of the water in the tank (it doesn't have to be completely empty but low enough to not make a mess) and remove the gravel (I've used a large dog scoop - the type for food). Then refill the tank and make sure it is the same temp as the old water (i use 33 gallon pails to set the water out over night - but you don't have to do that). Make sure to have remove the chlorine (I use sachem prime). Then re-add the fish. You can put the water in the pail into the tank but you don't have to.
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Also i put down a plastic drop cloth (i actually use a huge garbage bag split to double the size) before i start to catch any gravel or water that misses the garbage. Be careful how much old gravel you put in a container and remember you will have to carry it - so if you have 50 lb of gravel in your tank make sure you can carry that much and your container is strong enough to hold it.
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Also i put down a plastic drop cloth (i actually use a huge garbage bag split to double the size) before i start to catch any gravel or water that misses the garbage. Be careful how much old gravel you put in a container and remember you will have to carry it - so if you have 50 lb of gravel in your tank make sure you can carry that much and your container is strong enough to hold it.