Glowlight Tetra Swimming Strangely!?!

[icepop]

I just went to have a look at my fish tank and I noticed that my glowlight tetra is swimming very strangely. Two days ago I did a partial water change and replaced the activated carbon insert for my filter.

Just tested the water and here are my readings. Seems like the tank is cycling again?

Temp: 28 C (82 F)
pH: 7.4 (is usually less than 7.0)
Ammonia: 0
Nitrite: 0.3
Nitrate: 15

Could the tetra's swimming behaviour be a result of the tank cycling again? All the other inhabitants of the tank (head and tail light, black neon, cardinals, and rummy nose) are swimming normally.

I am worried. He is swimming almost vertically at times and seems "out of it".

 

[valdok]

tanks normally don't cycle again... how long as your tank been running? is his stomach "bloated" or bigger than normal? your nitrites and nitrates should be zero after cylcing. how big is your tank? and how many fish do you have in it? i don't want to guess before i know what the scenario is. Drastic changes in pH aren't good for fish, in general, either.

 

[Luca]

Nitrates actually rise 'after' cycling, not before, and your nitrates are a little high, they should be around 5-10. If your tank cycled recently it could be from a nitrite spike.

Try a water change, keep water conditions as stable as possible. keep us updated,
goodluck.

 

[icepop]

I did a water change and even trying to isolate the tetra .... but it's condition has worsened severely. It's nearly dead, lying at the bottom of the tank, struggling.

As I am relatively new to the hobby, this is the first fish I've ever had to see suffer like this. I feel just awful ... especially if it's something I've done wrong to make it suffer this much.

I found some information online that said glowlights are particularly sensitive to nitrates and nitrites ... but that's about all I could find out. I suspect that may be the cause. The other fish seem fine but I worry that the same might happen to them. *sigh*


EDIT: Since my tank originally cycled, all the levels in the tank have been 0. So I suspect my changing the carbon filter is what changed this. How do you avoid this kind of situation?