There is a special kind of siphon for cleaning an aquarium. It will pick up the gravel momentarily, then drop it, but the debris is lighter in weight and gets sucked out.
Here is a picture of one.
http://www.fosterandsmithaquatics.com/p ... catid=3889
It is a large tube (size varies, they are 1"-2" diameter x 6"-12" long) connected to a 3/8-1/2" x 4' or longer piece of vinyl tubing.
Put the end of the vinyl tubing in a bucket, use the large end inside the tank. My fish are so not-scared I have to keep them away from the tube. They want to see what is happening in there.
Start the siphon, and plunge the end into the gravel. As the debris and gravel go up the tube watch to see the gravel does not get too far up the tube. Most of the time the gravel will just fall back to the bottom by itself. Sometimes you might need to pinch off the vinyl tube to slow the water flow and allow the gravel to fall back.
Lift the large tube and plunge it into another spot in the gravel. Repeat.
You can hover the siphon near a decoration while you pick up the item to clean under it. The siphon nearby can pick up some of the debris that is disturbed when you lift the item. Then vacuum the gravel under it.
If the tank has not been cleaned in a while you might be removing more water than you want to to clean the whole tank. Just do enough to drain perhaps 25-30% of the water from the tank. Next time, do another area, then another... until the whole tank has been cleaned. Many people find they can do about 1/3 to 1/2 of the gravel with one water change. They will alternate, cleaning the left half this week, the right half next week...
Have enough water prepared to refill the tank. Dechlorinator, any mineral additives (mostly for hard water tanks) or whatever else is needed.
By working on the tank a little at a time each week your fish will be able to hide in the other half while you move rocks or driftwood and vacuum under these things, and each half of the tank will not get too dirty between cleanings.
38 tanks, 2 ponds over 4000 liters of water to keep clean and fresh.
Happy fish keeping!