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what are you planning on keeping in the tank? a 55 can be a little harder to stack rocks in because you dint have the width that you do on a 75 but you can look through your lr and pick you vav pieces to keep on top. try to get some base rock for the bottom and stack accordinly If you are going to keep many fish try to make a few hiding spots for them but be careful to make sure that the rocks are sturdy and wont fall on top of them.
setting the rock directly on the sand bed is fine. you may want to make an open area for the fish to swim and a seperate mound of lr. the stick on thermometers are fairly accurate and for the most part should work pretty well.