The Emperor Is A Tyrant

Your post is well timed as I have just added another 10 emperors to my 180 litre tank, making a total of 13.

I had just 3 males in this tank as that was all the shop had in. Maybe it is because they are males, or because I've only had them a couple of weeks - or maybe it's the bigger tank - but I've had no problems with these fish. Perhaps it is when there are both sexes that problems arise. I will certainly be keeping an eye on mine.

They are reputed to be peaceful; the only things I found during my research was people who had only one, or maybe two or three, having problems with aggression. Seriously Fish does say

One of the best tetras for the ‘general’ community setup. It is lively, peaceful and its colours offer a pleasing contrast to those of many other species. It’s a good tankmate for most commonly available livebearers, danionins, rasboras, other tetras and peaceful bottom dwellers such as Corydoras or smaller Loricariids. It can also be kept with the majority of commonly available gouramis and dwarf cichlids. Obviously it isn’t safe with larger species that may see it as food. In a South American-themed tank as described above, it can be combined with other peaceful tetras, pencil fish, Apistogramma and other dwarf cichlids.

Always buy a group of at least half a dozen of these, preferably 10 or more. It’s a shoaling species by nature, and will fare much better when in the company of its own kind. It actually looks far more effective when maintained like this anyway. Rival males are territorial with one another to an extent, but no serious damage is done during their battles provided the tank is sufficiently large and well-decorated.

Something I had better ask - do you have Nematobrycon palmeri or Inpaichthys kerri? SF comments that the latter can be nippy. Having seen both species in shops, I kerri were more difficult to sex with the difference being just the colour of the adipose fins. Mature N palmeri males have trident tails and different coloured eye rings from females.


But this doesn't help your problem.
Moving just the bigger one will cause the rest to sort out a new pecking order, which may or may not result in the same problem recurring. And the bully could well pick on the fish in his new tank - something I've read happening when there are only one or two in the tank.
I wonder if moving the bully and a few females (if you can manage to tell them apart) would be the best plan, with the idea that if he was the only male he would concentrate on the females without rivals.

Other than that, I don't really know what to suggest for the best 

That's what I'd be inclined to do. I know all the literature says to keep them in big groups, but some fish just don't want to go by the book.

If yours all have blue eye rings, they do sound like males. The first three I got were definitely males - trident tails, blue eye rings etc. In the second shop yesterday most of the emperors in the tank looked like females. There were two or three with blue eye rings, the rest had pale green eye rings; more of a greenish silver, nothing like the strength of colour of the blue in the males. I couldn't go by the trident tail as even the blue eyed fish had nipped tails.
One other difference I have noticed - blue eyes have a yellowy border to the anal fin while green eyes have a much thinner one to none, though it could be that my new ones are immature and have yet to develop the border.

And I paid double that price for my new 10!

I've split them into two groups of five. The five biggest have gone into a bigger (120L) tank where there are a lot more "active" fish to distract them, plus it is very heavily planted so that there are more places where they can easily get out of each other's line of sight. There is a fair amount of posturing and display, but because they are all about the same size (and all males) nobody can totally intimidate any of the others.

The five smaller ones  have gone into a tank with ten Silver tip tetras and three Arched Corydoras. When I put the first two in, they were immediately surrounded by the Silver tips, not biting but literally pushing them around. I  thought it wasn't going to work out, but after a short time they seemed to settle down and now each tetra species basically occupies one half of the tank. There's still room for more stocking in that tank and I'll be looking to add another type of tetra, hope they will all get along 

I think part of the problem was that the original tank wasn't big enough for the number of Emperors and I doubt if you will have the same problems in your big tank. In addition, that tank only had Panda cories as the other fish, while in their new homes there are other active mid water swimmers that seem to keep them distracted from focussing so much on each other. Before, the smaller ones were literally quivering wrecks, cowering in the bottom corner among the dwarf Cryptocoryne and getting chased back in as soon as they poked their noses out.

For a while, I thought I only had eight as that was all I could find in spite of searching the tank as thoroughly as possible. Because of the dense planting, it would be quite possible for a fish or two to die without me noticing. However, that evening I noticed another small Emperor moving very cautiously about, looking as if it could hardly believe that it wasn't being attacked. I put in a bit of food and it went berserk, swallowing very piece it could find in a few seconds. I suspect it hadn't eaten for a very long time. The next morning, I was looking for this small fish but instead I found another fairly large male! Where this one had been hiding before, I have no idea. He was staring at a small "cave" under the bogwood and I could see the little one hiding in there, which is where I suspect it had been hiding before. And so I do still have 10 Emperors and eventually did manage to get them divided. So far so good, but it will be a while before I'm totally confident that all will continue to be well.

Their original tank now only has the Panda cories, but I'm about to move 6 Checkered barbs in there and there will still be room for half a dozen of another small shoaling fish, yet to decide what.

When Sue mentioned her emperors earlier this week, I'd been wondering how yours were getting on. Thanks for the update - helpful advice for others who may be considering that species of fish.

A further update -

The largest male is about 5.5 cms excluding the tail, so somewhat larger than the generally indicated maximum.

I now have a clue as to the agressive behaviour of the dominant male. A while ago I noticed two very small fry had appeared in the tank, quite elongate with a pale line along the side. I thought they were Cherry barbs but today I had my first good look (they hide away in the plants) since they have grown larger and I realised they are actually baby Emperors. Another look showed me that one of the larger Emperors is actually a female so that I had 4 males and 1 female in the tank. I now suspect that the largest male had bred with the single female which would explain his highly territorial behaviour. In addition, I have just spotted another tiny fry which suggests that they have bred at least twice!

This is a very busy time for me but in winter I have more time and will be looking to reorganise some of my fish and will have to think how to best accommodate the Emperors.  Both Thinkfish and Seriously Fish suggest that a 60 cms tank is big enough for a shoal of these, but from my experience I would definitely not recommend this. I do have two 60 cms tetra tanks, one with 10 each of Lemon and Flame, one with 10 each of X Ray and Glowlight and they are absolutely fine, but with the potential size and territorial behaviour of the Emperors I reckon something like Sue's arrangement is much more suitable.