I've been keeping it between 68 and 72. Can you change out the rocks with sand once a tank is set up? He burrows in the rocks, and I know that's not good for his skin, but I'm scared to try changing them out and putting sand in at this stage in the game. There's salt in the water, but how much would you put in a 12 gallon tank? It's not enough to even qualify as brackish I don't think. Also, do you know how big fugu can get? My lfs said about the size of one of those fat Gatorade bottles, but they also told me to keep the water around 80 degrees.