Danios and Tetras Lock-Jaw?


I have a problem that has developed over the last couple of days that no-one seems to be able to answer.

I have a 40 gallon tank for freshwater tropical fish. The tank is relatively new and just settled down and cycled, so I thought I would introduce some fish.

I added some small fish, including some newly acquired Zebra Danios as well as some other tetras that I already had in another tank. I also added two Gouramis, 5 neons, one male and one female betta and two Angels (all purchesed this week)

I bought the danios from a reputable source that I have used before without any problems. However, within two days, one of them died with the following symptoms:

Locked jaw; unable to close

skimming the surface at a 45 degree angle

irratic behaviour; swimming frantically around the tank, bumping into other fish and plants etc, then spending minutes just floating round totally disorientated

Then it died.

Over the next day I lost another two by the same behavior pattern. I did a 50% water change straight away, treating the water with Tetra AquaSafe and Tetra EasyBalance. I was advised by the local pet shop that they had been fighting and this was the result of that. Last night, one of the other tetras went the same way.

Today I removed the remaining Danios from the tank and placed them in the other aquarium. they are all very lively and chasing each other around, but appear to be healthy. Half an hour ago, I lost another tetra by the same method, which leads me to the conclusion that there is a desease in the new tank.

Has anyone got any ideas as to what it may be, and how to treat it?


First off, I don't think its a god idea to put two bettas in your tank, especially with gentler fish like the danios. Are you sure when they are chasing each other around, spme aren't actually getting picked on. If they are this can lead to stress, and they will mostlikely die. When my aunt had big fish tanks but wanted smalll fish in them shed divide it up by either putting livebearers in there, or tetras, or chichlids. My point is some fish are not good to mix with others. I have heard danios can be put with bettas, and other aggressive fish, because they are fast. I wouldnt be as concerned this was happening (because maybe it was just the danios were bought sick), but now that a tetra died I don't know. Was the gouriamI chasing the danio or the betta chasing it. Because if it was most likely they weren't playing. I looked around on the internet its really hard to find a reasoning to lock jaw. MY opinion is this stress possibly from the other fish, (if the aggressive ones seem to be chasing the smaller ones). Or maybe (if you are feeding them pellets), the danios and tetras mouths are too small for them ..making them go to extremem measures.. I don't really know what lock-jaw is though I tried to find out hopefully someone else here will get back to you soon
  • Thread Starter


thanks for the quick reply and the ideas.

The bettas and gouramis don't seem to be bothering the other fish at all, and in fact, all the tetras tend to school together.

The tank is 4'x12"x15" with a capacity of 40 gallons. The filtration is done through an air filter box and a Fluval and air is bubbled through a 9" airstone. The temperature is maintained at a constant 25C (78F).

I feel a little concerned that if there is a disease involved, then it now affects not one but two tanks, and that could be catastrophic!

I had never had tropical before, as I tended to stick to cold water, but the tank came at a good price complete, so thought I'd give it a try.

One thing that may be worth mentioning: Not all the new fish came from the same retailer. The Danios, Neons and a pleco from one shop, the bettas, gouramis and angels from another, and the tetras came from yet another some weeks ago.

The other thing I'm doing differently is feeding them bloodworms as a supplement to the tropical flakes, and algae pellets for the plec. (Sorry, forgot to mention him earlier, but he migrated from the other tank at the same time as the tetras and I tend to forget he's there!).

Their appetites seem fine and they all get stuck in when the food arrives. I have also noticed a bacterial bloom in the tank today, and algae starting to settle on the glass, leading me to nitrates thoughts.

I did another 25% change of water today, which should help to alleviate this problem, and will continue with this for another few days on a daily basis until the bloom goes.

I am inclined to think that I may be looking in the wrong direction though. Any more suggestions with the new info?


:-[hmm..can't really find anything wrong from what you are saying...oh, well I assume youve checked the fish and there is nothing on them like ick..or some other illness..Have you cycled the tank? also how many fish do you have all together? and were the danios and tetras put in there last? any other fish scting strangely still in the last few hours?
  • Thread Starter



Since separating the tetras, I have lost another two fish, and a fungus appeared on the ornaments in the tank, so I guess I had some sort of fungal invasion in there.

Also, the two I lost were bettas, not something I particularly wanted to happen!

Yesterday morning I took the very dramatic action of putting all the fish into my little 2' tank and completely emptying the big tank and cleaning the whole thing out.

Bit of a squeeze in the small tank though... I ended up with 6 goldfish, 2 angels, 2 gouramis, 15 neons, three tetras, three danios, 2 plecs and 2 bettas, all trying to get on in a 15 gallon aquarium!

The immediate thing to do, was to get the big tank back into action as quickly as possible, so after taking 5 hours cleaning and boiling all the different bits, I managed to get it back in place, re-filling it with treated water using tetra AquaSafe and being very cruel, quickly heated it using the boiler and the tank heater, and transferred the 6 goldfish and the 3 danios into the clean tank.

Before you start, I know it hasn't cycled, but with the crowd in the other tank, I didn't have time for that luxury. I am just going to have to keep changing the water daily till it cycles with the fish in there. However, I also added tonic salts to the water this time, to make sure that any fungus that might have survived is taken care of. The tank temp is now at 30C, (I also did some reading and this is apparently the best cure for fungus), which the goldfish LOVE! I never considered the goldfish as enjoying warm water, but they love it, playing in the bubble streams and the powerhead jet.

As for the other tank, I seems to be stable and the remaining fish have enough room for the time being. Luckily, they are all small at the moment, so there is enough room until the big tank is ready for them again.

For the future, I am getting a small tank for quaranteen this week, so that this problem won't occur again.

Thanks for all the advice. I think I've learned a few lessons here, not least, to ensure that the fish are well and compatible BEFORE I introduce them to a large tank, and not to buy too many fish too soon!

P.S. I also found out that the source of the infection was not the danios, but the bettas, that my partner bought from the local pet shop the next day. Out of the 3 she bought, all have since died. If I had the QT the problem would never have arrisen, since they would not have been introduced to the community tank at all. Gives you a good reason to get a quaranteen tank, doesn't it!

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