sick salamander, please help

[aspen1]

Hello, I have been keeping a group of several adult marbled salamanders (Ambystoma opacum) in a 15 gallon tank for about 3 years. The substrate is a couple of inches in depth, composed of a soil-based mix with some added silica sand to improve drainage (pebbles beneath this for drainage), a few plants, a shallow water bowl and some flat shale rocks that they use as hide spots... they have hearty appetites and eat large numbers of small crickets, tiny earthworms when I can find them, etc.

This arrangement seemed to work quite well for a while, but recently we have had a serious summer heat wave in this area, and I couldn't keep the AC on all the time. Also, I think that I was preoccupied with a number of things, so unfortunately, I neglected to change the substrate for some time, and there may have been a buildup of toxic waste products in it... One day suddenly a couple of them started acting strangely, pacing about the tank uncomfortably, so I immediately changed out the substrate, and they seemed to be OK after that.

However, one specimen became extremely weak, barely able to move, and I had to isolate it in a separate small container for observation. After about a week, it is still alive but seriously ill; it apparently is bloated, with some cloudiness in the eyes, and the rear legs seem to be paralyzed.

I am realistic enough to realize that it probably has very little chance of survival but as of right now it still seems to be hanging on, able to move its head and front legs, wiggle its tail, etc. Is it best just to euthanize it, or is there something I can do to save it? Any suggestions would be tremendously appreciated...

I would also like some suggestios as to what type of lighting is required to successfully grow mosses, small ferns etc. in a woodland tank setup... I am constantly trying to grow these types of plants there but my success has been extremely limited.