Fresh water fish for 40 gallon tank?

I was going to get a pet iguana but now it's seeming that it would take too much care and maintenance. I have a 40 gallon tank and I want to get one or two (a few, 3) at the most fish to put in it. I need to know what kind of rocks or gravel needs to go in it and what my options are as far as fish. Any help?
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