Thanks Miscel for replying!
Yes, they do get enough sunlight. In fact, a few weeks ago, it had developed brown leaf-tips, so I had to move it to a little shade. I guess, they are over-watered.. since the soil did not dry up since a week.. I will punch more holes in the container today. Or should I move them into a smaller container? I dried to lift them, but the roots seems to spread throughout the container...

I saw some little white insects and fruit flies in the soil too. How do I get rid of them? I just added some chopped garlics now...

Sounds like over-watering. Since the roots have spread, it might be easier to punch more holes in the bottom. If you do plant the mint in a smaller container, I wouldn't worry about losing some of the roots as the mint plant is pretty resilient. In the meantime, you might want to try a cutting as back-up.

Whiteflies - I made a yellow sticky trap using vaseline, a popsicle stick, and a piece of yellow paper. I tapped on the stem to dislodge the whiteflies which flew into the yellow sticky trap. Weirdly, after I made my sticky trap, I had some vaseline on my fingers. When I was putting the stick in the soil, I noticed some on the mint leaves. Using my finger, I just lightly touched as many whiteflies as possible and caught quite a few. Haven't had an issue since, though I did spray some homemade insecticidal soap on the leaves a few days later for another pest issue which I assume helped get rid of the remaining whiteflies.

Given the size of your mint, you might want to hold off on the insecticidal soap as it might be too much for the plant. I accidentally killed a small mint cutting that I had planted in soil. The soap solution (a drop of dawn, a drop of olive oil, water in a small spray bottle) was too much for the cutting which was doing well until then.

I think some ppl spray/mist water on the leaves. though I didn't.

I will punch holes today, and be careful about the watering. I hope it gets well soon... I will wait till tomorrow, if the fruit flies dont go away, I will apply the trap... have a great time!

Your mint is also desperate for light. Can you open the blind? Or put it outdoors?

If that plant were to be placed in the direct sun, it would fry in a very short time. I'm afraid that moving it to a much brighter location might not be worth it.

The brown tips were caused by excess moisture, not too much light. The fruit flies are fungus gnats, another indication of chronically wet soil. If the 'little white insects' were squirming around in the soil like little worms, they are the larvae of the fungus gnats. Were they wormy or flying around like tiny white moths?

Here's one more opinion about your future options. Purchase some new potting medium, add extra perlite, if possible. Find an appropriately sized container and prepare to repot by removing the plant and gently shaking off as much as the old medium as possible and trimming the roots with sharp, clean scissors.

Then, cut back the top nearly to the soil level! Repot, water in thoroughly, and relocate in a very bright, sunny location. Mint thrives in the full outdoor sun so we need to accommodate them indoors.

With suffucient light and much umproved water management on your part, the new growth should be full and healthy. Of course, you must provide nutional support to any containerized plant.

Use your fingers to feel the moisture content of the medium before watering ALWAYS. It should be fairly dry deep inside the container.

I would be remiss if I didn't mention to you that most culinary herbs will never be happy inside, not really. This forum is packed full of stories just like yours.

Thanks! They were quick in moving (not like worms), some were like fruit flies, some were just little insects moving fast in the soil.. Yes. I did the mistake of overwatering once and the soil did not dry up for a week. Now I added garlic into the soil and kind of shuffled the soil.. the fruit flies have reduced.. if not totally gone ... what kind of fertilizer should I give to the mint?

Yes - of course it would need hardening off before being put outside but I think it could cope with the blind being raised for starters.

@floral_uk, I do not have any outdoor space. So i have raise the blinds :) . There was no more damage, although the leaves are small and stems are leggy ...