New Penguin Tetras Aggressive And Apparently Eating Each Other

Hello,
I'm a beginner, and have a new tank. I did a fishless cycle - see details of tank, and the cycling below. Once my tank was cycled, I bought 6 penguin tetras. All 6 seemed fine in the shop tank. By the time I got home one had bits nibbled out of its tail. I acclimatized them, then put them in the tank. By morning there were only 5 fish in the tank. No signs of Penguin 6 remained. I thought he was perhaps hiding and perhaps sick, so I searched under the ornaments/plants for him. No sign of him in the tank at all. Then another has disappeared last night... They are all feeding fine and swimming around the middle-top of the tank. No-one is gulping air at the surface.

But they are all very aggressive and bullying. I cant see that its one individual, seems to be all of them gang up on the weakest.

I know I should have more than 4 (or even 6) - I bought all the stock they had, and I didnt want to overload my tank initially.

My questions/dilema now is, do I go and get more penguin tetras to try and balance out the group, or do I leave them as they are and hope they calm down/stop eating each other?!

Tank Details: Love Fish 64Litre, with internal filter and led light as supplied with the tank. 2 Java ferns on bogwood, anubias on coconut shell and amazon frogbit floating. Did 6 week fishless cycle. The final week both ammonia and nitrite all zero after daily dosing to 1ppm ammonia. Nitrate levels rose to 80 at end of cycle, but I did a lot of large water changes before getting the fish and it was below 20 ppm after these. I took water from the tank to the shop when I bought the fish, and they tested it and confirmed it was OK. Temperature constant at 25oC. No CO2 enrichment. Gentle bubbles from an air stone at the back of the tank.

Many many thanks Sue! I did dose upto 6ppm early on - the bottle slipped!  then I held it at 3ppm for a week or so, then went down to 1ppm for the last week. Hopefully not too many of the bacteria died off with me reducing it.

Hi Alwyn and welcome to the forum.

I have penguin tetras, 10 in total, in a 125L tank, along with 10 glass bloodfin tetra and a group of silver hatchets.
Originally I also had a bristlenose plec in the tank, but had to take him out as I saw one of the penguin tetras nipping his tail.
I like the penguins, but agree that they are a bit nippy. I still sometimes see one with a bite out of it's tail, but they don't seem to be aggressive towards the other tank inhabitants, and even leave the shrimp alone.

Welcome, Alwyn and sorry to read that you've lost a couple of fish.

On the basis of personal experience, it might be worth checking the filter, behind the filter and in the filter bracket, even if you don't think the fish squeeze in there - just in case there's any possibility that they've ended up in there. One of mine ended up there, most likely during a chasing/mating game, so I plugged up the gaps with filter wool to ensure that it doesn't happen again.

If adding more fish, it might be worth getting more females if you / the staff are able to sex them in the LFS. Dispersion of aggression and dispersion of attention towards the females by having more of them may help ie the males may feel less like the need to compete if they are "plenty of female fish in the sea". 

Penguins are know nippers, especially when they feel cramped. Unfortunately your tank is too small for them.

Regarding your cycle, you did the right thing. If you don't get fish straight away you need to keep the bacteria fed. A couple of times in a week would have been enough, but daily does no harm.

You can cycle with 1ppm or 5ppm or anywhere in between. It all achieves the same result. Even cycling with 1ppm will give you more bacteria than you'll need even when the tank is fully stocked, and most will die off after the cycle due to the drop in ammonia supply.

Penguins are know nippers, especially when they feel cramped. Unfortunately your tank is too small for them.

I would say you'd be fine with a couple more fish at least - http://www.seriouslyfish.com/species/thayeria-obliqua/ (considered a well-renowned international resource) details and tank dimensions would seem to suggest that - and see how you get on. Fingers crossed that this helps.

  Great news! I went last night to get more penguins, and they've settled in wonderfully, I'm very happy to report. There are 9 in total now. They're all schooling together, all feeding well, some are out hunting the Cyclops that hitched a ride into my tank on the plants, much more like the behaviour I was expecting, and everyone is MUCH calmer!

So relieved! 

(And so far water chemistry is holding strong. I'll be keeping a close eye on it though)

Really pleased to read this and hope that it continues in this way.