Bully Danio

[Spybreaker]

I'm having some trouble with a bully Zebra Danio. I've had this fish for over a year and his companion recently died (an unfortunate filter accident). Instead of getting another Danio to keep him company, we opted for a little Platy. After adding the Platy, I started noticing some aggressive behavior from the Danio, so we talked to the local fish shop and they recommended getting 2 more Zebras. So...I added 2 young Zebra Danios and hoped they'd school and ease the original Danio's stress, but...instead he's going crazy and terrorizing every fish in the tank. Now he's the only one that swims around the tank...all the others (we also have 3 Neon Tetras) just hide. I'm especially worried about the Plat, who has taken to hiding completely motionless under a plant for long stretches of time. The only time I ever see the Platy move around now is when the lights in the tank are off...or if I'm trying to net the old Zebra.
 
Any suggestions?
 
Thanks!
 
Tank size: 10 gal
pH: 6.5ish
ammonia: 0
nitrite: 0
nitrate: 0
kH: Don't remember
gH: 180
tank temp: 76
Volume and Frequency of water changes: 20% about once a week.

Chemical Additives or Media in your tank: Have only added stress coat since getting the young Danios and seeing aggressive behavior from the old Danio.

Tank inhabitants: 3 Neon Tetras, 3 Zebra Danios, 1 Platy

Recent additions to your tank (living or decoration): 2 Zebra Danios and Platy

Exposure to chemicals: None 

 

[eaglesaquarium]

This isn't a case of a bully fish.  This is a case of improper housing.
 
 
Danios are ACTIVE fish, and they are a shoaling species.  IMHO, they shouldn't be kept in a tank smaller than 4 feet long (not because of their size, but their activity level).  And they should only be kept in groups of 6 or more - generally... more is better.  
 
 
This fish is likely 'acting out' because of the tight confines it finds itself in.

 

[Spybreaker]

Thanks for the quick reply, eaglesaquarium. I'm a bit curious as to why the tank size never seemed like a problem before. It's only recently that this Danio has shown such aggression.
 
Assuming the size is the problem...and I can't currently go any larger with the tank...what can I do to solve this problem? Do you know of any sites that would help me give the 3 danios to someone with a larger setup? I think Craigslist has a policy against listing pets and I'm not sure where else to go. I don't really want to kill this fish, but he's seriously terrorizing his tank mates.
 
Thanks again.

 

[eaglesaquarium]

Hard to say why the danio didn't act out earlier.  But, I can tell you that they are extremely 'active' and just that activity could be enough to scare off more timid species.
 
 
You could look for some local aquarist clubs.  They could help you with that.

 

[StevenF]

One problem I observed with these fish when I had them is that store didn't just have Zebra Danios.  they also had long fine zebra danios and leopard danios.  And since they were all juveniles you couldn't tell them apart.  However as they grew it became apparent that I had two regular zebras, one leopard and one long fine zebra danio.  The leopard and long fine were significantly larger than the other two and the and were very aggressive to the two smaller ones and other fish.  I had to get rid of them.  I also learned that some fish farms sell "mixed" zebras to the stores.  The smaller ones were peaceful to other fish.

 

[NickAu]

 
But, I can tell you that they are extremely 'active'

That is an understatement Eagles LOL, I have 12 of them in a 6 foot 100 gallon tank and cant imagine them in a 10 gallon tank.

 

[DWC]

I have a shoal of 6 Zebra Danios in a heavily planted, sponge filtered 10 gallon tank and they have been active and healthy for well over 2 years. Happy? You'd have to ask them. They started as a shoal of 8 but 2 passed on right away. The remaining six are fed once daily with Tetra flakes, pellets or freeze dried bloodworms. Their colors are a brilliant royal blue alternating with gold or silver depending on individual. I do 30% weekly water changes and the ammonia and nitrite are always 0, nitrate 5-20 depending on testing methods (strips or drops).

They DO have a definite pecking order and chase each other but no physical harm to themselves or tankmates (4 albino cories and a cherry shrimp). Would they be better in a 20 long, or 30 long, or swimming pool? Assuredly they would but not all of us have the space or means. They can thrive in a 10 with proper care. I would advise heavy planting along back and sides to break sight lines for "chased" fish to escape to and INCREASING numbers to 6. You can always get a rogue individual who just doesn't get along (still talking fish here).

David

 

[Byron]

I just came to this thread, and may be able to offer some help for the future if not the present.
 
Taking first the "proper" environment...like all "danio" (as commonly named in the hobby) they are active fish, and that means they should have lengthy tanks.  Seriously Fish recommends 90cm/36 inches as minimum.
http/www.seriouslyfish.com/species/danio-rerio/
And in a group of 8-10 minimum.  Obviously the fish, like so many, can manage in smaller tanks and fewer numbers.  But for the benefit of members reading this thread who might take things out of context, I would point out that our aim should be to provide the optimum conditions so the species can thrive, not something with which the species may "manage."  There are better fish species for small tanks than active swimmers.  And as the zebra danio can (should) reach around 2 inches, it is not a "small" fish in the first place.
 
To the matter of an individual fish suddenly turning nasty, this is almost inevitably due to the environment.  For years many in the hobby have recommended minimum tank sizes and minimum number for shoaling species, but only recently have ichthyologists begun to actually look into how such things impact fish.  The results are not surprising, but they do give credence to what many have maintained all along.
 
Shoaling fish (and the study included angelfish, a couple tetra species, and something else I can't remember off hand) housed with less than five experienced stress that increased to the point where the fish became aggressive.  Naturally aggressive species like the angelfish (aggressive here meaning territorial hierarchy, not physical tearing each other to pieces normally) became significantly more aggressive, and "peaceful" species like black neons became aggressive.  The conclusion was that the fish were under stress from their inappropriate environment, and aggression was about the only way the fish could fight back out of frustration.  Sometimes the opposite may occur, and a fish will withdraw and simply waste away.  But the aggressive response was prevalent in the study, as indeed it is in the hobby.
 
Once the stress has reached the level where the fish is forced to somehow deal with it, the result is not reversible.  "Fixing" the problem will not restore the fish to normal behaviour.  The damage of stress is physiological, and permanent.  This is why we always advise getting suitable numbers at the outset, and housing them in sufficiently-sized tanks.  Some species do not take long to establish an hierarchy--angelfish for example--and once the dominant male in this example has taken up his territory, which will be the entire tank in most cases, the dye is cast, so to speak.
 
I don't know how many times I and others have advised such and such for numbers and space, and the member has come back with "they have been fine for "x" weeks" or similar.  The problem is that most aquarists cannot possibly know what state the fish are in, mentally.  Sometimes outward appearance and behaviours provide a clue, sometimes not.  Sometimes fish react significantly, sometimes not.  Individual fish, like individual animals of any species including humans, can vary quite a bit.  The safe approach is always to assume the species will be "normal" and provide accordingly.
 
Byron.

 

[Liv15]

I had the same problem when I had 2 zebras in a 48L. Upgraded to a 3 foot and bought 6 more and it stopped. They are community fish but because of their naturally boisterous behaviour, they need plenty of their own species and space to interact with. Other species of fish can find that their normal behaviour is too much.