Listless Black Phantom Tetra

[KCB]

I noticed today that my eldest female BPT is acting very listless and was just floating about the bottom of the tank, as if she just didn't have the energy to move

. She is the only one acting this way and so was placed in my hospital tank. She was acting normal yesterday evening when they were fed. Here's some details:
 
Tank size: 46 litres
pH: 8.0
ammonia: 0ppm
nitrite: 0ppm
nitrate: 40/80ppm
kH: 15
gH: 16+
tank temp: 24/25

Fish Symptoms: Listless, heavy breathing, slightly wider than usual but no pine coning

Volume and Frequency of water changes: 25% every week

Chemical Additives or Media in your tank: Just dechlorinator.

Tank inhabitants: 4 other BPT's and 4 Endler Guppies

Recent additions to your tank (living or decoration): None.

Exposure to chemicals: None.
 
My tank was fully cycled (fish-less) in January and has had fish in since then. I don't know what to do really, it's possible it is old age as I rescued her from another person and has been in my care for a few years now. I don't think it will be a water chemistry issue as she is the only one with symptoms and I've never had an issue before (the tank is understocked as well).
 
I have some anti-internal bacteria medication but I don't want to use it unless that is the issue.
 
Please help, I'm completely at a loss 

 
 
P.S: She is the fish in my avatar.

 

[techen]

Without test results it's hard to tell really what it could or could not be.

 

[KCB]

Right, some results are back:
 
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: Not back yet but looks like it will be 40/80ppm.
 
This was using an API master test kit.
 
EDIT: Yep nitrAte is 40/80ppm (hard to tell with this kit).

 

[techen]

Sounds fine to me, If there back to acting normal it could have just been a phase or old age.

 

[KCB]

Here's a picture of the fish and the results:

Sounds fine to me, If there back to acting normal it could have just been a phase or old age.

 
I guess time will tell, thank you for your help techen.

 

[techen]

Those nitrates are kinda high, What is in your tap water?

 

[KCB]

It's a little darker in that photo than what it really is, but yes it is a little high, I've never been able to get it lower despite large changes. In December it was coming out of the tap at 20ppm. I know I should be able to get it lower but I can't even put a dent in it, do you think I should do multiple changes?

 

[techen]

If your tap water was 20 then 40-80 isn't too bad. Your fish get used to what you put into the tank so I'd say the nitrates are too bad.

 

[KCB]

If your tap water was 20 then 40-80 isn't too bad. Your fish get used to what you put into the tank so I'd say the nitrates are too bad.

 
Thanks 

 

[KCB]

Just to update; she passed away in the night 

. I hope she had a good life in my care.