My Panda Cory looks hurt

Hi all,

this morning I woke up to find one of my Panda corys dead. She looked fine, no wounds or anything. I did tests on my tank and everything is fine.
I've just checked my tank again, and another cory is on her own, looks like she has trouble moving, and she has a wound above her mouth. What could have happened?
I've had them for 2-3 months without issues, other tankmates are Espei Rasboras, otocinclus, betta, bladder snails and assassin snails. The tank has been running since December, I stocked it in January and there have been no issues.

The betta doesn't go near them, he's really nice and just likes to chill and eat. I don't think he could have done that.

What can I do to help her? And to prevent this from happening again? I'm worried she's not eating and she's probably in pain and super stressed

No expert but the barbels look eroded and possibly infected. If it was me I would think that the cory has an infection likely from the barbels. I see you have aquarium soil, when I had cories on aquarium soil, their barbels got eroded and they did not do well. I would suggest you move the cories to a hospital tank that has a sandy substrate or possibly no substrate until the barbels heal. Others might know better what to do to treat the current infection.

Note when I look at aquarium soil under a microscope, Tropica brand I believe I had, I find a lot of sharp angular fragments which could cause damage to the cories mouth parts. I won't get into the sand vs. gravel debate but I do believe that aquarium soil, at least some of them, are very hard on cories.

@Uberhoust, okay, I'll move her, thank you. The other corys look completely fine.
Another thing I just noticed is that she is completely white, while the others are more like brownish. What does that mean?

As for the soil, I've got a quite heavily planted tank, what can I do at this stage?

I would suggest that the white color means she is not well. I will be honest with you I don't have a lot of good luck with cories, you have to keep your tank pretty clean for them otherwise they seem to be sensitive to infections. You have to consider you water parameters as well, I don't believe they like nitrate leaves much more than 5ppm.

I like the plant performance with the soil, but overall I haven't found that the fish do as well. I changed my large tank 75 gallon to a small gravel substrate, mostly angels in that tank, and my other tank with soil to sand. My plants in the big tank do not do as well with the gravel, but the plants in my 37 gal. with the sand substrate seem to do well.

I chose the route of not raising cories at the time, at least until I can devote more time to them. They are easily available, cute, but they are not the easiest fish to have thrive, at least for me.

I also found that as the soil gets older it is harder to maintain the tank as the soils start to break down.

Uberhoust is correct, the barbel erosion is due to bacteria in the substrate. This is common with any so-called plant substrate, and cories must never be kept with this type of substrate. They need soft sand. It can be the roughness, hard to tell from the photo, but the bacterial problem is more usual and likely what is going on here. Move all the cories to a tank with soft sand.

Do not use bare bottom tanks ever, this is just as bad because it is impossible to clean the bacteria from the glass and believe it or not the fish can have more barbel issues from glass shards.

The above advice is from Ian Fuller who knows more about cories than all of us put together.

I've got pandas. One thing is the substrate. They really prefer fine sand. Rough gravel aggravates the barbels.