I recently had a problem with aggressive guppies. We have (had) 6 guppies, 3 pairs. We're not pro's and don't even know their types, but two were yellow, two were red/orange and two were smaller & blue. All very pretty and active. We also have 6 Tetras in the sparesely planted 80L (20Gal) tank with an overkill filter. No population pressure here!

Both Blues were agro. Not full-on psycho's but constantly bugging the Yellows and bitting at their bottom fins and possibly anus. One of the Blues began this behaviour immediately (2 months ago) but it got gradually worse and recently both Blues were going for the Yellows non-stop and had caused bleeding in both. Interestingly they did not bug the Reds. If the blues went for the Reds, then the Reds (which are bigger) beat the crap out of them for a few seconds, then ignored them. The Yellows almost never fought back and even when they did the Blues just ducked for cover then came straight back.

I tried taking the Blues out during a water change, but the problem started again immediately. I then consigned one blue to the freezer. The second Blue did not modifiy his behaviour, so he too bit the dust. Now all is happy. I'll probably start to introduce some more guppies later. They're much more interesting than Tetras.

So stay away from the blue ones!