by Rockfish » Mon Jun 09, 2014 9:27 pm
You didn't mention actual numbers, but ammonia and nitrite should always be essentially zero, any detectable level could cause issues. High nitrates can cause problems over the longer term, but I wouldn't expect levels up to 50 ppm to cause any short term problems.
As you've had all the fish a while, something like gill flukes causing the gasping is probably unlikely. Bacterial infection might be possible. I wonder if the cories might have scavenged from the dead shrimp and picked something up? If it was just the one adult cory then you could put it down to an isolated mystery death which could have dozens of causes, but losing three over a few weeks could indicate a creeping problem. If it's not water quality then a slow-acting bacterial issue could be a possibility.
In the absence of any obvious cause, it's probably a choice between just maintaining good water quality and riding it out and hoping the deaths stop and the remaining fish are resistant, or trying a general anti-bacterial medication. It's not always ideal to use a medication without a specific problem to treat, but sometimes it can stop further deaths.
