Please post an image or two.

Sorry my reply hasn't been too prompt, I wanted to give my fern a break between the constant digging-ups. I feel like I might be hastening its doom everytime I dig the spade in.

Here's a mealworm and also some casings (there are heaps of casings all through the soil):

Here's a closeup of the leaf tips on my red-potted fern:

And here's a comparison between my red-pot fern and my fungus-gnatted-but-comparatively-healthier green-pot fern:

That's some sort of caterpillar, not a meal worm.

Nah nah, the picture is quite unclear and this particular mealworm was dying so it's body seems darker than usual, but trust me they're definitely mealworms. Also, none of them are eating the leaves or even above soil, I have to dig under the roots of the plant to get to the creatures.

These bugs have a hard exoskeleton and only three pairs of legs near their heads. Caterpillars have many more legs than that, and they're generally soft and squishy creatures.

Ok, but I was judging by the three pair of legs towards the front, the set of prolegs that I see, and what appears to be a pair of anal prolegs at the back. But, certainly, you have the advantage being able to see it in person.

( By the way, caterpillars only have three pair of 'real' legs, the rest are stubby prolegs.)

Anyway, it's probably some sort of darkling beetle larvae, usually called meal worms. All are organic matter or live plant eaters. They would certainly be of no benefit in a container plant's soil, where they would be tempted by tender plant roots.