fish dying with no symptoms - please help!

[wazzie]

Hi, I am new to the forum but desperate for some help!
I have kept fish for many years but I am at a total loss as to what to do with this situation.

Basically, in the last few months I have lost six fish in my 200l tank, without any sign of anything being wrong. I do a quarter change once a week with treated water which has also been left to stand overnight.

My tank has been up and running for about three years, it contains live bearers, danios and Corys and there have never been any major problems but in the last few months I have lost a catfish, a zebra danio, two male swordtails and a molly.

The water parameters are absolutely fine, the fish are not stressed, I haven't changed anything. They have all died suddenly with no symptoms, with the exception of one swordtail who became pale quickly and died within a few hours.

I have treated for parasites just in case but I have no idea what to do now!

If anyone has any idea, please help me! Up until now I have only lost two fish since the tank started so I have no idea what is going on.

 

[Fishfur]

How many fish do you have in this tank altogether, and of what species ?

 

[Cactuspixie]

Also, what are your results for ammonia, nitrites, nitrates and ph?
Have you changed or added any substrate, rocks, wood, plants, fertilizer etc?
Do you have adequate surface agitation?
What dechlorinator are you using? Your town may have recently begun treating the water with chloramines, which some dechlorinators don't detoxify. You don't need to stand your water overnight. I dose my dechlorinator for the tank's volume and fill the tank straight from the tap. If you are filling with buckets you can just dechlorinate each bucket and then put straight into the tank.

 

[NinjaTetra]

It could just be their time. Ive lost two fish eight months apart with no real symptoms except for floating sideways for about a day before their eyes glaze over, which im pretty sure isnt anything more than a constapation causing bloting in the stomach which squishes the air bladder, and they cant eat or swim straight. I would say its probably just natures way of doin what she wants to do. Dont dose if u cant identify it. The most likely thing is ur ammonia lvls may have rose (from maybe the first fish rotting for a few hours after dying from underfeeding or overfeeding), then the next fishs gills are damged from the brief spike of ammonia( remember, it would be sort of like slowing increasing the amount of, say, carbon monoxide, in an airtight room until it reaches a point where our organs **** down from lack of oxygen) so it could be a chain reaction of one fish just dying. Just a geuss. Im not saying u did somethin wrong, just that sometimes the artificial ecosystem we call an aquatium balances itself a bit.

 

[Fishfur]

If he got the fish when young and has had the tank only 3 years, none of the species he listed should be dying of old age as yet. Even zebra danios can live 5 years, mollies about 4 or 5, etc. He says he hasn't changed anything, so that's not likely the problem.

Old age also does not usually cause sudden death. Elderly fish usually slow down some, start to look a bit poorly, much like us, before they die.

One thing I am wondering if is it simply rotten luck with some fish that were inbred, perhaps some non visible genetic issues and thus possibly not able to live normal lives. Even those I would not expect to just up and die, but it is possible.. sort of like having a heart attack, perhaps.

 

[wazzie]

Thanks for your replies. The fish are mollies, platys, guppys, zebras and corys. I actually keep a lot of tanks and have done since I was a child which is why this is confusing me so much. It is a planted tank and I have been doing loads of water tests and they are all fine.
There is plenty of surface agitation , I have two air stones and the filter has a tube extention which runs across the surface in little individual streams.
I don't think it is old age because they are not very old really and I have lost fish to old age before and it was a slower process. I had a swordtail who was about seven and he passed away slowly, first becoming lethargic and pale.
I was starting to wonder if it was TB perhaps but I would expect to see symptoms at least in some of them.
Would anyone advise treating for parasites or bacteria or anything else, even without symptoms? I just cannot think of what else to do

 

[Fishfur]

No. I would not add any medication unless I was virtually certain of what I was treating. Based on your last post, I am leaning toward thinking it just might be you had some very bad fortune with some stock that was not genetically sound.

Inbreeding is becoming an increasing issue with so many fish today, particularly those being raised in large numbers on farms, which include most of the ones you named. They have no practical way to control which fish breeds with another fish, which leads to inbreeding upon inbreeding. Sisters with brothers, moms with offspring, dads with offspring, even when it is not done deliberately to fix some new trait that is found to be desirable, such as colour or finnage.

Feral fish are generally widespread in their habitat and only a few fry from any given brood typically live long enough to reproduce, all of which greatly lessens the chance of inbreeding and the possible expression of recessive or lethal genes. But even if the commercial farms bring in wild stock from time to time to broaden the genetic pool, it can only do so much in the face of the enormous numbers being bred.

Zebra danios in particular are bred in the millions, being popular lab subjects for a lot of research.

Since there are no observable symptoms, I would not do anything differently, other than perhaps wait some time before adding any more fish to this particular tank.