Do community fish establish a pecking order?

Question

i have a tank with three fish. a guppy and two mollies. the guppy is picking on the black molly but not white one. in fact i think the guppy tries to snuggle up with the white one. the black molly sees this and starts chasing the guppy around, and then the white fish. i am confused these are supposed to be peaceful fish. this is a forum post with pictures of the fish.

http://www.fishforums.net/index.php?showtopic=255694&pid=2115237&st=0&#entry2115237

Answer 1

Community fish do establish pecking orders.  It's up to you when you believe it's gotten out of hand.  Generally, if a fish is inflicting serious bodily harm or causing another fish to not eat for an extended period of time, the bully should be separated for a little while.  Do you know what s*x your fish are?  (I looked at your pictures and it's hard to tell the sexes.)  I wouldn't be shocked if you have a male guppy and female white molly.  Guppies and mollies are closely related (closer than guppies and platies or mollies and platies) and will try to breed, even though they aren't usually successful.  This snuggling that you have observed is probably the guppy trying to mate.  

Now, it's possible that your black molly is a male and does not like the guppy trying to mate with the female molly (just normal male aggression), and once it chases the guppy away it tries to mate with the female white molly.  )These suggestions are based totally on the guppy and black molly being male and the white molly being female.)  

99% of the time guppies and mollies get along quite well.  If they haven't been together for a decent amount of time, they're probably still working out their pecking order.  Someone's got to be in charge, even if you're dealing with peaceful species.  

Answer 2

if it's not harming the fish, it should be fine. many fish have pecking orders. clown and yo-yo loaches have a very definite leader. there's usually a dominant fish in a school (like tetras).  if the fish don't appear to have fins nipped, or one fish cowering in the corner, they are probably fine. you really ought to have more than just the 1 guppy, but it may not help much