Do you quarantine them? I had cories a while back, and some of them started looking poorly a while after I got them. I treated them for internal parasites, lost one and the rest were great.

 

@JJ09 Some were added directly to an established tank (when I was quite new to the hobby), some were added into a new tank (cycled, new stock being added). I haven't tried adding any long enough after my tank was set up subsequently to need quarantine, although if I do get more I will definitely be quarantining.


They generally lasted a few months though before starting to die off, and no other fish in the tank died during this period. If they were sick it wasn't something they were passing on; they just seemed to one by one get less active and then a day or so later be dead. It's been a while so my memory is a little hazy on the specifics.

What did you use to treat them? I imagine if I got more, it might be a good idea to go ahead and treat them regardless as a safety measure? And on that note - would it be a terrible idea to use the tank I'm setting up for a cherry shrimp colony, currently empty, as the quarantine? I don't know if the medicine might leave any residue if I ran carbon after.

 

Corydoras have very particular temperature needs that are rarely paid attention to. The majority need to be kept from 72-76 degrees long-term, but are often kept in temperatures higher.

The mistake people make is they see a temperature range online, like the peppered corydoras, for instance, that says 72-79 and make the assumption that the extreme of either end is fine long-term. It is not, The extremes of either end mark seasonal temperatures that the fish will be exposed to temporarily during a given year. The middle few numbers are the temperatures that are optimum to keep the fish healthy.
Temperature stress takes time to compromise fish, but eventually it does. With corydoras, when stressed, it is common that they die of Red-splotch disease and other bacterial related illnesses. When fish are healthy, they are in balance with this bacteria; but, when stressed this same bacteria can get the upper-hand.


If you look at Planet Catfish, one of the most reputable resources for catfish species on the web, they only list corydoras acutus for tanks with temperature over 78 degrees. Thats it. Ian Fuller, one of the most prominent ichthyologist researchers specializing in corydoras species states that none should be kept above 78 degrees. And, again, this means 78 being a temporary seasonal exposure.


These are the handful of corydoras that can take temperatures in the higher 70's better than others: corydoras sterbei, c. acutus c.adolphoi, c. duplicareous, c. splendens ( formally brochis splendens).

 

I've found something similar with pygmy corys - they'll fatten up after purchase and be really active, and then I'll start losing them one by one after a few months. Their colours go really dark and then they flop onto their sides and die. Out of the nearly 20 that I've purchased, only 2 have survived, who did get ill in the first "plague" but survived and now look great, although I won't be buying any more of course. All other tankmates seem to have been fine.

Interested to see who else has had the same experiences and what could be the cause. Mine were kept in very similar parameters to yours, except at about 24C (75F), nitrates at ~10ppm, and with slightly lower kH and slightly higher gH.

 

I've found something similar with pygmy corys - they'll fatten up after purchase and be really active, and then I'll start losing them one by one after a few months. Their colours go really dark and then they flop onto their sides and die. Out of the nearly 20 that I've purchased, only 2 have survived, who did get ill in the first "plague" but survived and now look great, although I won't be buying any more of course. All other tankmates seem to have been fine.

Interested to see who else has had the same experiences and what could be the cause. Mine were kept in very similar parameters to yours, except at about 24C (75F), nitrates at ~10ppm, and with slightly lower kH and slightly higher gH.

Take a look at this- there may be clues in here. Think of areas that may be stressors, which is basically the basis of all diseases in the aquarium.

Corydoras pygmaeus (Pygmy Cory) ? Seriously Fish

 

78-80 is definitely on the high side. I keep my Corydoras tank at 73-75 degrees. Try Corydoras sterbai if you can't lower the temp. I believe they are often kept with Discus because they are more tolerant of high temperatures.

 

I had peppered cories- I kept them in a room-temp tank which was usually between 68-72 degrees. When I suspected parasites (fish was pale and listless with stringy, colorless poo) I medicated the tank with API General Cure (praziquantel and metronidazole)

 

I'll try putting my temp down to 76; I don't really need it so high now that I'm not keeping the rams at present. Hopefully with that, and with giving them a pre-emptive parasite treatment in quarantine, they should do alright.

 

I had a similar issue and keep Discus. I lowered to 76 and both species are doing fine. I have Panda, Sterbai, and Julii Corys.

 

@Discusluv - thanks for the link, I appreciate it! I use seriously fish a lot, and everything there matches my tank. Looking into kH and/or parasites now but was just interested to see if anyone else had similar issues and if they knew the cause. Thanks again