Help! "highlander Syndrome"

[FryDaddy]

I've tried to pick the most peaceable fish I can, starting with a load of neons in my 30 gallon freshwater tank (about 8 fish, added after cycling for a few months with danios). They were stable for a few months, then one decided to become so aggressive, he killed all the others one by one.

I have had similar problems with zebra danios, guppies, and now mollies. It doesn't matter if I isolate the aggressive guy, another rises to take his place very soon. It is like once they get their mind made up to start being aggressive, they all get into battle mode.

I've got some driftwood decor for them to hide in, and the tank is understocked. Water quality seems fine, I even have a few fry in the breeder tank at the moment and they're doing fine. This last round, I had four dalmation lyretail mollies, (3 were male, which may have contributed), and the female died while in labor. After that, the boss male killed the medium male, and now he's after the smaller guy. I don't know what to do other than isolate him.

Any advice is appreciated! It is so strange, just when I think the tank is stable after adding new fish (a couple of months), they start this again! Arrgh!

 

[glolite]

How long has the tank been running for? If it's newly cycled then neons will die due to ammonia/nitrite poisoning. They are normally added to an established tank after 4 to 6 months.

 

[FryDaddy]

How long has the tank been running for? If it's newly cycled then neons will die due to ammonia/nitrite poisoning. They are normally added to an established tank after 4 to 6 months.

This tank has been running for 2 years, no problems with nitrogen... well-cycled for a few months with danios, before moving to other things. It isn't so much just the neons, I've had this trouble with about 5 species of fish now. They'll be stable for a couple of months, suddenly I'll come in to a death, then I'll notice an aggressive fish attacking the others, and possibly another death. If I isolate him, another rises to take his place, until they're all dead or quarantined. By the way, I've got four neons in there while the mollies have done this, and the tetras are fine/left alone. It is just within-species that there's a problem! It is very frustrating.