Maybe this is just my academic side speaking but what about typing her up a little info sheet that explains what a cycle is and the need of tropical and cold water fish. Maybe also include a list of compatible fish for a 5 gallon, 10 gallon, and 20 gallon, in case she decides to upgrade eventually. This way, if she's bored she can pick it up and read it. Also, you don't have to tell her you made it. Just be like" Oh, I just visited this great new fish store in (blank) and they had these great handouts. I grabbed an extra one for you because I know your interested in fish etc. blah blah blah"