Whats your readings for ammonia & nitrite?
How long has the tank been set up?
did a water change this morning he has seemed to have lighened up a little bit after that but theres still something wrong with him
Before Water Change
Ammonia - 0.25ppm
Nitrite - 0.25ppm
Nitrate was a little high at 10ppm which is why a did the water change
(was told i dont need to worry about Nitrate as much as the Ammonia and Nitrite but i still do)
After Water Change
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - between 0-5ppm
tank has been going for about 7-8 weeks now only added fish less than 2 weeks ago
Nitrate at 10ppm is a relatively low figure - many people have more than that in their tap water.
In the 5-6 weeks before you put the fish in, I'm guessing you weren't putting any ammonia in the water, either as pure ammonia, or as something like fishfood which would break down into ammonia?
Have a read of
this and
this.
A precis of this is that your filter needs to build up colonies of two sorts of bacteria, which will process the ammonia produced by your fish into nitrite, and then into nitrate.
Until you've got enough bacteria, you need to keep doing water changes big enough to keep the ammonia and nitrite levels below 0.25ppm until you are available to change the water again. In other words, it's not enough to change enough water to get your levels down to 0.25ppm, because within a short period of time, the fish will have produced more ammonia, and your levels will be over again. You need to be testing the water at least daily to check the levels of the poisons, and be prepared to change the water at least daily to keep those levels low. 20% per week isn't going to be sufficient.
You're relatively lucky, in that you've only got two fish, so the amount of ammonia being produced is relatively low. However, because you have a small tank, the concentration of ammonia and nitrite will quickly rise.
I think the reason your fish is behaving as it is, is because it is suffering from ammonia and nitrite poisoning. That's why it looked better after you'd changed the water.
If you don't keep these levels low, it's rather similar to humans smoking. Sometimes, you see a human coughing on the smoke. Sometimes you don't. But damage is being done to the human's lungs whichever. Same with the fish. One of your fish showed symptoms. The other didn't. Damage is being done to their gills whichever.
Good luck!
