patches of colorless scales

Question
I have two veiltails in a twenty gallon tank (charcoal filter). The tank was set up last spring with live plants.  The fish are 3 1/2 years old and 7-8 months old. I change water 2-3 times a month (5-7 gallons).

I feed them fresh food (cooked): peas, spinach, broccoli, cauliflower, celery, sweet potato, mango (not cooked), etc. with occasional protein (worms, egg yolk) and grains (rice, bread crumbs).

This morning I found both hiding under plants and acting very sluggish.  The large one had two areas on his body that had colorless scales (not Ich).  I immediately checked the ammonia, nitrates, and nitrites levels and they were all zero.  I then changed 50% of the water, replaced everything in the filter, and added stress coat thinking that somehow some poison might have built up in the tank.

This evening they are doing better, swimming around and loooking for food, but they are still sluggish and the large one still has the colorless patches.

Do you have any idea what this might be? Could it have anything to do with the temperature? The tank is unheated and was about 63 degrees this morning. Or could it have something to do with the ammonia chips I put in the filter?

Any help you can provide will be very appreciated.

Answer
Hi Sylvia;

I'm so sorry it took so long to answer your question. My computer crashed. It's up and running again, so here we go...

It sounds like the temperature got so low that they just couldn't do much. Hopefully they are doing better now. It's also possible something startled them when you weren't around and they scratched themselves on something in the tank. Fish can jam themselves hard against the gravel too and cause scale injuries. Let me know if anything new has developed since you first wrote me...

At Your Service;
Chris Robbins