So I have a 72 gallon tank for my foot long fire eel and all of the substrate is gravel with one tube and several rocks for the fire eel to hide under. I have noticed that the fire eel started burying himself under the gravel after the first week that I had bought him and I read that they can get abrasions from the sharp edges of the rocks which can cause diseases. I bought a bag of sand and im trying to decide exactly how to go about setting my tank up to have a half gravel and have sand mixture.