Over the last number of days a number (2-3) of our peppered and panda corydoras have died, and this morning another one is lying very lethargic at the bottom of the tank, alive but only just
All of the other fish in our well-established 120l tank (various barbs and tetras, albino corys, shrimps...) appear to be fine and active, the water tests (ammonia, nitrate, nitrite) are all spot on (I have been doing regular water changes).
It appears that *something* is affecting just the corys. The ones that have died have had no visible marks or scars anywhere and no other effects such as bloating and the 'only just alive' one, apart from having its fins closed down, appears visibly OK. I am attaching a photo.
If anyone could suggest anything I'd be really grateful; the only thing that comes to my mind, having racked my brains, is the slight possibly of overfeeding, but nothing as changed and why would only the corys be affected?
Test results: Ammonia: 0ppm Nitrate: 0ppm Nitrite: less than 0.3mg/l Tap water is slightly hard Temperature: ~24'C
We took out one Panda Cory weeks ago which had damage to its tail fin, as we didn't want to risk any potential problems spreading to the other fish. We’re not sure whether it was fin rot or nibbled by other fish, though we haven’t added any new fish for a bit, and it’s a community tank, so we don’t tend to have nipping problems. We have noticed that occasionally shrimps will 'pester' the cories as they lie 'asleep' on the bottom of the tank, sitting on top of them and appearing to 'nibble' at them until the cory 'wakes up' and swims away. I've taken another photo of the remaining 'poorly' cory as I'm not at all sure that the dorsal fin looks right?
We haven't noticed any damaged barbels. We realise that gravel isn't ideal for cories but we've had them in the same situation for as long as we've had the tank (>5 years).
It seems odd that all of the the panda cories and two pepper cories have died within days of each other, yet the albinos are fine - touch wood!
There's nothing obvious from what you've said, except that 0 nitrate rings alarm bells. As the final product of the cycle, it's almost impossible to have 0 nitrates in a stocked tank. That may indicate that the cycle failed recently and produced ammonia and/or nitrite spike which killed the fish.
Fishlady wrote: There's nothing obvious from what you've said, except that 0 nitrate rings alarm bells. As the final product of the cycle, it's almost impossible to have 0 nitrates in a stocked tank. That may indicate that the cycle failed recently and produced ammonia and/or nitrite spike which killed the fish.
The 'zero' reading for nitrates is probably not 100% correct - our API Nitrate test kit has colour bands where yellow is zero and the next slightly orange band is 5ppm. The last test (taken soon after a regular 20% water change) was nearer to zero than to 5ppm, hence I put zero in my last post.
My wife has just been reading an article online which has made me think ... the tank has a lot of plants, wood and stones, which is a good habitat for the fish but makes it an absolute pain to clean the gravel. A week or so ago I did a 'deep clean' - moving all the stones and wood and cleaning all the gravel, which stirred up quite a bit of debris which the cories, by their nature, were foraging amongst. Do you think that it's possible that they have been 'poisoned' by being exposed to this debris? Should I be being more meticulous with cleaning the gravel? I have read elsewhere that having a build up of bacteria in the gravel as well as the filter is beneficial?
We've also been considering (prior to this disruption) fitting an external filter in place of the standard Fluval internal unit which has served us well since we bought the tank; obviously that won't make cleaning the gravel any easier but would you consider it a worthwhile upgrade? Would having an an external filter make the tank any "healthier" than an internal?
The majority of beneficial bacteria are housed in the filter so there is no need to avoid disturbing the gravel, in fact, if detritus builds up in the gravel anaerobic bacteria can thrive in the gravel and produce toxic hydrogen sulphide gas which is released into the water when the gravel is disturbed. If that is what has happened it may account for the deaths. There is a characteristic "bad egg" smell when hydrogen sulphide is present. There's an article here about this problem: http://www.fishyportal.com/cgi-bin/pub/diag?c=v&id=60
You need to syphon the gravel as part of your weekly water change to prevent a build-up of detritus and so also prevent a build-up of hydrogen sulphide gas.
As to filtration, if your current filter is controlling the ammonia and nitrite there is no need to upgrade to an external, but you can if you want to
Thank you so much for your help and advice. As I kind of expected the remaining panda was hiding under a stone this morning. When I moved the stone it clearly wasn't at all well, so for the benefit of the other fish I have euthenised it I have also now done another clean of the gravel and changed 25% of the water. I think I just need to be more conscientious with my cleaning and regularly move the rocks and wood to clean underneath. Fingers crossed that the remaining fish are OK. Thanks again, George