Make a gravel cleaner from a 1, 1.5, or 2 litre plastic drink bottle and a garden hose or length of clear plastic hose.
Cut the bottom off a plastic drink bottle and throw the bottom bit in the recycling bin.
Remove the lid and plastic ring from the top of the bottle and put those 2 bits in the recycling bin.
Stick a garden hose in the top of the bottle and run the hose out the door onto the lawn.
That is the gravel cleaner.
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Get a couple of large plastic bins or storage containers and put them near the tank. Fill them with tap water, add dechlorinator and aerate them while you gravel clean the tank.
Get a small water pump like an AquaClear 402 powerhead or similar.
Attach a length of clear hose to the outlet of the powerhead and run it to the tank.
Make a U out of pvc pipe and attach one end of the U to the end of the plastic hose. The U gets turned upside down and hung over the side of the tank.
When the tank has been gravel cleaned and drained, put the pump in the bucket of dechlorinated water and turn the pump on. The water goes from the bucket, up the hose and into the tank. The pvc U holds the hose in place on the tank and lets you move around and turn the pump on and off without having to hold the hose in the tank.