Hi, i already have 3 tanks set up with guppies and Mollys (one tank with fry, one with females and another tank with males), today i bought a 75ltr tank, it is set up and will be cycled before adding fish. My daughter wanted to use marbles instead of gravel (so i know this may rule out some fish). What can we put in it? would like something different but also be able to use it if my fry tank is full (for older fry) if we have too. Any ideas? (no tetra's though).