Just over a week ago I bought one extra albino corydoras for my aquarium to add to the group of four in the aquarium. The albino corydoras that I added turned out to have part of it's tail fin missing and it was relatively pale and still compared to the rest of shoal. Today, as predicted, the corydoras with a segment of it's tail missing became deceased so I removed it immediately on spotting it.
From today onwards I have noticed that the rest of the group of Corydoras were continually going to the water's surface for air every 30-40 seconds. For some reason, they are taking oxygen directly out of the air rather than obtaining it through their gills. This alarmed me quite a bit as I immediately thought that there may be a lack of oxygen in the aquarium, perhaps due to there having been a deceased fish in the water for possibly 10 hours or more. However, the CO2 drop-checker indicated that the dissolved CO2 content in the water was safe and the other species of fish, black neon tetras, were not hanging at the water's surface for air.
I decided to do a water test to see if another parameter was out of range and found that the nitrite level was between 0 and 0.25 ppm. Given the result, I immediately did a 85% water change and observed the behaviour of the corydoras afterwards. I noticed that they were not going to the surface at all for a good 2 hours and then all of a sudden they started the old routine of going to the surface for air every 30-40 seconds. If this wasn't perplexing enough, the next thing problematic issue I noticed was that the drop-checkers (I have 2 on either side of the front glass pane) were still indicating the same CO2 reading. The 85% water change should have turned the solution in the drop checkers to a blue colour, however, the colour remained green!
Given all of the above, should I come to the conclusion that I've gassed the fish and the filter sponge bacteria (nitrobacter and nitrosomonas) with too much CO2? I know filtration bacteria need oxygen too to convert ammonia to nitrite and so forth so the presence of nitrite would suggest that something has upset the filtration bacteria.
Anybody know why the Corydoras are still constantly taking oxygen directly from the air?
From today onwards I have noticed that the rest of the group of Corydoras were continually going to the water's surface for air every 30-40 seconds. For some reason, they are taking oxygen directly out of the air rather than obtaining it through their gills. This alarmed me quite a bit as I immediately thought that there may be a lack of oxygen in the aquarium, perhaps due to there having been a deceased fish in the water for possibly 10 hours or more. However, the CO2 drop-checker indicated that the dissolved CO2 content in the water was safe and the other species of fish, black neon tetras, were not hanging at the water's surface for air.
I decided to do a water test to see if another parameter was out of range and found that the nitrite level was between 0 and 0.25 ppm. Given the result, I immediately did a 85% water change and observed the behaviour of the corydoras afterwards. I noticed that they were not going to the surface at all for a good 2 hours and then all of a sudden they started the old routine of going to the surface for air every 30-40 seconds. If this wasn't perplexing enough, the next thing problematic issue I noticed was that the drop-checkers (I have 2 on either side of the front glass pane) were still indicating the same CO2 reading. The 85% water change should have turned the solution in the drop checkers to a blue colour, however, the colour remained green!
Given all of the above, should I come to the conclusion that I've gassed the fish and the filter sponge bacteria (nitrobacter and nitrosomonas) with too much CO2? I know filtration bacteria need oxygen too to convert ammonia to nitrite and so forth so the presence of nitrite would suggest that something has upset the filtration bacteria.
Anybody know why the Corydoras are still constantly taking oxygen directly from the air?
? If there is any ammonia / nitrite in the tank, the Corys often are the first "to notice" being lowest in the tank. 
