Illness/Sickness: Paddle-tails not eating & frequently skin shedding

I haven't posted on here for over a year so everything's been fine for my newts until now.

I have two paddle-tails (I believe a pachytriton labiatus and pachytriton brevipes - I thought they were the same at first but now I firmly believe they are different. They were kept together without fuss in the pet shop though) and they act fine together and I've only ever seen aggressive behaviour between them once (over food. I feed them in a seperate container now partially to eliminate this and also to improve water quality in the tank so there's no decomposing food). Plenty of hiding places for them in their tank. I also keep a Chinese Fire-belly in a different tank and he's in good health with a voracious appetite so no problems with him.

For the last week and a half neither of my paddle-tails have been eating, turning away from food (frozen bloodworm which they always eat) and I have noticed their shed skin caught in the filter. I have seen them shed their skin before but even after clearing it from the filter input, shed skin reappeared every second day or so. They newts seem to be hanging about the filter input as well, possibly because it helps suck the skin off them. They normally stay in their respective hiding places (a plant pot and an aquarium ornament) and occasionally come out, so this is very unusual behaviour.

I change 1/3 of the water every week (using dechlorinator) and the filter as well, and if I ever see the temperature approach 20C then I drop ice packs in to bring the temperature down, though it's Scotland in March so that's not really been an issue. So I'm struggling to think of it as a water quality issue. I feed them frozen bloodworm twice a week and they always stuff themselves, but neither will touch them just now, even if i wriggle the food in front of them.

At first I thought they had just eaten enough for once and weren't hungry but after this length of time that's not the problem, and the overly-frequent skin shedding has me worried too.

If anyone can help this would be extremely appreciated, I just hope they don't have a parasite and haven't caught anything though this seems to become an increasingly worrying possiblilty. Also thank you for reading this far, I wanted to fully explain so that answers could be as concise as possible and wouldn't state obvious errors eg water quality, not the right food

What kind of filter are you using? Do you have any plants in the tank?

The filter is something similar to this, though possibly a different size:

Interpet Internal Power Filter PF1 Internal Aquarium Filters for Sale

The tank is a 60x24x36 so it's adequate. The newts crawl under and behind it.

And no there's no plants in the tank, I had plants months ago and the newts just ripped them up and they floated and started deteriorating. I did consider getting java moss though.

Update: I've tried digging in the garden for worms to see if the newts will eat a different food, especially if it is live, but unfortunately I only found one tiny worm no more than an inch long in half an hour of digging. I will try again tomorrow and maybe order some from livefood to see if they will eat this as the small worm was actually eaten by one of my newts.