What is worrying me
One of my peppered coreys appears to be having difficulties with balance/buoyancy and possibly breathing. When I got home from work she was swimming rapidly from one end of the tank to the other at surface level, she was also gulping for air, but what was most unusual was that she was swimming at an angle and appeared poorly oriented (she swam into one of the snails). About 15min later she went to the bottom of the tank and wedged herself against a log for over half an hour. During this time the angle she was resting at got more and more pronounced until she was almost upside down. When I next checked she was swimming around quickly again. Now she is lying on the bottom on her side.
The behaviour looked quite similar to when a former goldfish had a constipation related swim bladder problem. We lost two fantails early on so we have been adding fish very slowly. We got two new peppered corys about two months ago and lost one of them about a month ago. I didn't observe any unusual behaviour, injuries etc before she died and water parameters gave no clues.
What I've done so far
I've fed peas to all residents which I generally do every one to two weeks, but I've read that this is unlikely to help with corys. I did a 25% water change with a gravel vacuumed two days ago but I checked my water parameters anyway and they are all what they usually are (pH 7.4, ammonia & nitrite 0, nitrate less than 5ppm) I don't have a test kit for other parameters. I don't have a thermometer, the tank is not heated. I've looked, but I can't see any signs of injury or infection. Her gills did look a little red about a week ago and she was a little lethargic so I did an extra partial water change and we skipped feeding for a day. I generally do a 25% change every two weeks.
Other info
The tank is a 50L planted tank with a gravel substrate. The filter is the one which came with the tank.(http://www.masterpet.com.au/products/product/?pid=EG538)
It is stocked with 1 black moor, 2 peppered corys, 3 apple snails. The tank has been set up for about six months now. Water parameters have been stable. All other inhabitants including the other peppered cory who has been in the tank from the second week after we set it up.
