PlanetCatfish.com

OK. Not the first time this has happened. It seems to happen when introducing new corydoras into the shoal. The new arrivals are sometimes found dead with a large red blotch on the side of its body, near the belly. The others are unaffected.

Water parameters.

Temp 24C
Ammonia and nitrites 0 mg/l
Nitrate 25 mg/l
GH 3
pH 6.8

I introduce the fish slowly, and put a half cup-ful of aquarium water into the bag every 15-20 minutes for an hour to an hour and a half before introducing them into the tank.

What is the cause of this angry, red blotch and how can I help prevent it in the future?

Sounds like a case of the dropsy. There is not a lot you can do to against dropsy disease.
It is very hard to cure once it has taken place on the fish.

Best thing to do is to check how long the LFS have the fish in the store and look closely at them before getting it.

Some people prefer to get them once they arrive in the store (to increase the growth rate) while others would wait to make sure they are somewhat of a healthy batch.

I know that the fish were in the LFS for at least a month. As I say this is not the first time this has happened and it does not seem to infect other fish in the tank.

I have heard of this red blotch on corys before in that when they die in transportation, a vast majority seem to exhibit this mark.

Melafix might work. See this thread.

Sounds like a case of the dropsy

To me the description given does not sound like dropsy.

If the only external characteristic is a red patch then the most likely cause IME would be a bacterial infection, but could depend on if the mark was seen before or after death?

How quickly after introduction did you find the cory dead?

Usually within a few days and certainly less than a week.

I read in a book somewhere that when corys are imported in, those dead are found with the same red patches on the side of the belly region. It was an old book, but by a respected author though I can't remember who at the moment. I'll look later. The book put it down to the suggestion that the dorsal and pectoral fins of corydoras hold poison and it is the concentration of this that causes many deaths in transit!!

You should see this thread, then.

xsu-xsu

What species are you having the problems with.

There are some species that are more prone to this problem than others.

Ian

c sterbae, semiaquilus and fowleri mainly but have had it with metae and a couple of others that I don't remember

Silurus
That is the book that I have and read this. I will re-read it and quote extracts tomorrow

oh, loxozonus was another and 'asher' (reynoldsi?)