Aquaponics 20: Failing at Aquaponics II

Hello all,

Things are not becoming the lush paradise I was hoping for...
Plants are growing fine, looking green and great, but only for a week or so and then they just give up on life and crumble.

Examples:

One day they are fine, the next they look like the above sorry specimen.

I've compiled a list of things I think could be causing this problem:
1) lack of sunlight? It is pretty dark here, but I do not think the plants would behave this way if it was a question of light.'
2) the water level or similar - the roots are wet and, again, wouldn't they turn yellow instead, if lack of water was the problem?
3) Temperature. I guess this is what is killing my plants. At the moment we are getting temperatures of -12 degrees and even though they are inside, the window is cold and the room temperature cannot adequately combat the temperature right next to the windows (especially as the heater is below the other window..).

So would plants suffering from a cold behave the way my plants are?

All the best,
Kasper