Sick Golden Panchax

Question
We have a 180L tank which is coming up to 4 yrs established. In the tank are Platies, Cories, Harlequins and a Pleco. We also have a pair of Golden Panchax which we've had for about 8 months. The female is currently ill and I can not find what is wrong with her, can you help? She has become discoloured (dark grey) down a third of the front of one side of her body, is sinking to the bottom of the tank, lethargic, not eating, and when she tries to swim, appears tail heavy and swims in a vertical motion. I have been treating the whole tank with a general antibacterial solution but she is still deteriorating and now has clamped fins. She has no external wounds or growths. I've tested the chemistry of the water which is fine, and the temperature is normal, all the other fish appear well as well. We gravel vaccum the tank each week, carrying out about a quarter water change. The only thing that has changed recently is that we introduced a Pleco a fortnight ago and changed the carbon filter a week ago, with her becoming ill a couple of days later. Do you have any idea what can be wrong with her and what I can do?
Thanks
Lorraine

Answer
Hi Lorraine:  it sounds like two possibilities... one is that she may be carrying eggs and going to spawn.  If she is going to spawn you should look around the tank somewhere near her for eggs... sometimes they lay them on plants or in gravel... if she has spawned then  you should remove the fish from the tank or the eggs whichever is easiest.... They tend to eat the eggs and or young....

The second possibility is that she is being harassed by another fish... They can be a shy fish so if she is being bullied you can separate her from the rest and see if she improves... fish that are bullied often end up sick from the stress they go through... if she perks up after separation then you have your answer.  It is possible that she is sick though I find that unlikely... Give the tank a good look and let me know if you find any eggs... or if you don't... hope this helps...dave