Help! What is this white patch on my swordtail?

Tank
75 Gallons
Been running for 8 months
Bubble filters, no carbon.
Has two heaters, set to 78 F
Tank contains 17 female swordtails, all sisters that have been with us since birth 3 years ago. Also 1 Pleco, 2 Yoyo Loaches, and a bunch of trumpet snails we're constantly trying to get out of there.

Maintenance
Water changes once a week, about 33% each time.
Replacement water treated with water conditioner.
Substrate is vacuumed each week, and filters are also cleaned.

*Parameters - Very Important
Tank is cycled.
What do you use to test the water? - API Liquid test kit
Ammonia: 0
Nitrite: 0
Nitrate: about 5 ppm
pH: 7.4-7.6

Feeding
Fish are fed with a mix of pellets, flakes, and veggie pellets once per day, 3 days a week. 2 other days they get cooked shelled peas, the other two days are fasting days.

Illness & Symptoms
We've had this fish for nearly 3 years.
She started getting thin a few days back, that we noticed.
Other than being thin, she's got a white patch on her back, near the top of her. It's on both sides, symmetrically. It doesn't look like anything external, like a growth or decay, but rather looks like something inside her has become whitish, or less transparent.
We started treating for Columnaris a couple days ago. We put in KanaPlex and Nitrofurazone. Yesterday was just Nitrofurazone, and then a few hours before we saw her with the white back we did a 25% water change and then put in both KanaPlex and Nitrofurazone again.
In addition to being thin and having the white patch, she's pretty lethargic. I'm guessing that's to be expected from her being so thin, though.
One of her sisters has been piping for the last half hour. We're considering doing a water change just to get some of the medicine out. We dosed with 0.75 grams of Nitrofurazone in the 75 gallon tank. It's the third day in a row of that dose. KanaPlex is according to instructions. We figured on 70 gallons of actual water, so we put in 14 scoops.

I don't really know what's going on, but I'm doing my best to find out. I thought that maybe someone here might have an idea. If you do, please comment. Any help would be greatly appreciated. Thanks all.

 

IMO, the white patch looks too symmetrical to be Columnaris. Fungus usually covers the tissue destroyed by Columnaris. It's apparently not present in this case.
The Swordtail evidently has some variety of intestinal problem, which might be age related.
I don't believe antibiotics will help in this situation.
Wish I could be more helpful.

 

IMO, the white patch looks too symmetrical to be Columnaris. Fungus usually covers the tissue destroyed by Columnaris. It's apparently not present in this case.
The Swordtail evidently has some variety of intestinal problem, which might be age related.
I don't believe antibiotics will help in this situation.
Wish I could be more helpful.

Thank you for replying.
I did end up doing a water change last night to get the medicine out, so it's a relief to hear that at least one other person thinks the antibiotics weren't going to help anyway. She's still here this morning, so that's a relief. I'm going to feed her soon and hopefully she'll eat. She did yesterday.
I still wish I knew what this was. I searched for hours online last night, and couldn't find anything that looked the same.

 

It could still be Columnaris but in a slight way. But sometimes older age may also cause lacking pigment in the skin tissue.

 

She hasn't eaten today. The edges of her tail are now brown. I saw one of her sisters picking on her when she was just trying to relax. I got her into a separate tank with Methylene Blue and Aquarium Salt, and she seems to have taken to it alright. I don't know what else to do though.
I doubt it's only old age that's causing the lack of pigment. It came on too fast for something like that, I'd think. Maybe though. I really don't know.
I know A201 mentioned an intestinal problem, but I don't know how that would be affecting the color of what I think is a muscle.
Any further insights are extremely welcome, and thanks to those who have replied so far.

 

It's always harder to give a good diagnose if the problem is something internal. Even a bad blood flow can cause coloring of the skin. But this would also be a speculation.