I have a 20 gallon tank for "mild-mannered" tropicals (and just recently started a 30 for semi-aggressives). I had a beautiful tequila sunrise guppy in my 20 gal. which was getting beaten up by 3 aggressive zebra danios (WHY?--question #1). Hence, I moved them to a 30 gal., for now. I got another beautiful yellow, spotted male guppy (thinking I was getting the original male guppy a friend), and during the night the new guppy completely dessimated the original guppy's tail fin, pulling it out in chunks and leaving only shreds (WHY?--question #2). He died that day. Also in the tank at this time we have two neon tetras, one female platy (bought the same day as the new guppy), one grey skirt (very shy), four African dwarf frogs (small) and one otto. Our mollies died previously, white skirts brought back to the store previously for trying to kill grey skirt. Please help if you have some ideas on this.