Be prepared to take it slowly. As you remove the gravel, you will be
removing some of the nitrifying bacteria colony. Monitor your ammonia and
nitrite levels as you proceed. You may also stir up great clouds of crud

He might want to think about overfiltering the water during the change. An
extra cycled filter would help with the bacteria, and mechanical filtration
in the filter would help with the crud clouds.
-Cesium


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