Need advice on fin nipping please...

[Pandora]

Okay - here's my problem.
I have a 9 month old 10g tank. 1 male betta, 3 albino cories and 1 julii have been tank mates since the start. About two weeks ago we introduced 4 guppies to the mix (2m,2f).
The betta for some reason hates the julii, but as he has plenty of places to hide (plants + small cave) he is fine, and as I said, they have been together for nearly a year.
My problem is that in the last three days I have lost all but 1m guppy, with the 3 dead guppies turning up dead with fins badly nipped. Seems to be happening overnight, as I haven't seen any aggression at all with the guppies. (and yes, I watch the fish for hours....and hours). The remaining guppy is very healthy, no nipped fins.
TODAY, I have found one of the albino cories has been nipped. I do NOT want to lose any more fish, but have no idea who the culprit is. I suspect the guppy, but I have have always thought they were peaceful? and have never seen him bother or go near the cories at all. The cories get along fine with each other, hang around together, no probs. At times the betta hangs around with the albinos - seemingly for company, NO aggression ever towards the albinos (which is odd - because he really doesn't like the julii).

Okay, sorry that was so long, but does anyone have any idea on the culprit.

[Sarkazmo]

It's really hard to say for certain... typically with guppies (as with most live bearers) you want a ratio of 3 females to 1 male to keep sexual aggression down to a tolerable amount. The guppies could've set off the Betta. Each Betta is different so it's really hard to say whether he found the guppies to be viewed as he would another male Betta. I don't have any experience with Julis so I can't comment on them. Being that the corys stay on the bottom I'd say it's probably either the Betta (which you said hangs out with them) or the Juli. Guppies WILL kill each other though. I've seen it. They're not the placid, peaceful fish that so many seem to think they are. I don't think I've been much help but hopefully I'll spark some insight.

Sark

[Maestro]

Hey Pandora, sorry for your recent losses.

I am going to agree with Sarkazmo's notion that the addition of the Guppies set off the Betta. In my experience, several of the Bettas I had over the years in community tanks have been occasionally violent to new tankmates. I haven't placed Bettas with other longed finned fish like Guppies, for fear of the Guppies being attacked. However, I had no problems keeping Bettas and Cories together.

I suggest you dose Melafix (if it is avaliable in Austrlia) to prevent and heal any infections in their wounds.

[terrapin24h]

bettas are said to have excellent color vision, which is one of the ways they recognize and attack other bettas. Your guppies are probably a variety of pretty colors, which are most likely ticking off the betta. Once he gets cranky, he probably figures to take out some aggression on the other fish in the tank. I'd relocate the betta.

--chris

[rich311k]

It is the betta. Guppys and bettas generally do not stock well together. The betta sees a rival male and attacks.

[Pandora]

Hmmm. I have a new 120L tank waiting to be picked up, but I wanted to hold off till Christmas. (it is a gift as the fish are technically my sons). If things don't improve in the next day or so I might have to pick it up early.

It is such a pain that adding guppies changed the dynamics of the tank so badly. The betta has never bothered the albino corys before at all.

Thanks for your thoughts.