Illness/Sickness: Health declining .. bad skin and gills.. not eating

Hi everybody,
I'm hoping someone might be able to help as I fear it maybe too late for my Jelly Bean.
She is a full grown golden axy, I don't know how old as she was full grown when we adopted her. We have had her for over a year and she used to be fat and active.
About three months ago we got her a friend, a black axy called Lickorish.
He was quarantined for a month and then joined Jelly bean in her 4 foot tank. He died on Christmas eve, it was very strnage as he had been eating fine ( better then jelly bean) and swimming around fine and happy, the next thing we knew he was dead that evening.
Ever since he jioned Jelly bean she stopped eating... we thought she was sulking as we had seen her have a few snaps at him, never bad just to tell him to give her some space.
Anyway - he's now gone, and her health has continued to decline.
She is much thinner, won't lunge for food, you pretty much have to poke her in the face with the food before she takes it and sometimes not at all.
We feed her a mixture of earthworms, mealworms, crickets, and pellets... depending on what she will eat. At first we thought she was just spoilt and holding out for the crickets. But now she won't even eat them.
I checked the water today and all indicators were fine, or with in reasonable levels. She gets a 50% water change every week with treated rainwater.
She is on a fine sand and sandstone substrate with two live plants and one plastic one and two largeish logs.
Her gills lost the filament nearly totaly and they have turned a deep purple colour in the last three days or so.
Her skin is white and scalely, and she quite often has the base of her tail raised/ near her bottom.
I checked she poo'd today and all. I'm starting to think maybe she is just getting old?
It's making me sad.
I'm really not ok with giving her a salt bath as I have never held her before - I feed with tweezers.
Oh I should have said we have a full filtration system, plus an airbubble thing... my huband put in ... doesn't seem to bother her, in fact when we took it out she seemed to look for it. We also have a refrigeration system that keeps the tank at a constant 19 degree Celsius
At this point I can't see us keeping any axy's in the future as whatching her health decline has been very upssetting for my 5 yr old daughter.. we have done everything we can think of and it has cost us a pretty penny or two.
If no-one can help me I will take some pictures and post them here.
Thanks

I'm trying to upload pictures of her but I am having difficulties....
think I may have sorted them out.
If you could have a look and tell me what you think, that would be great...

I can't think of anything, but a list of your parameters might be helpful. I usually read the sick axolotl section for my own benefit in case I need any of the info. When you say the parameters are within reasonable levels I'm assuming your refering to nitrate only. There should be no ammonia/nitrite at all. She doesn't look like she hasn't been eating, and if this started when you added your axolotl I would guess it's a disease transfered to her by your axolotl. Quarentine should have made any apparent illnesses clear, but maybe it wasn't an obvois illness, and that could have been the reason it died so early? I'm just throwing out ideas. It looks like a lovely setup.

Aww, Jellybean!

Keeping the thought that it could be related to poor Lickorish aside, you have a few pebbles dotted around the tank, could any have gone missing? Is her tummy always as rounded as that? (My axies are still quite young, so I'm not experienced with adults and their shapes) She could have swallowed one of the smaller stones, which might explain the tail position, if shes impacted?

I'm sorry I can't help any more!