Assassin snails behaving strange

[eicca]

I've got two assassin snails, had them for a month and a half. Initially got them to help control the population of bladder snails, which they have been doing splendidly. There are still bladder snails around, so food should not be an issue, and I've also seen them nibble at the food I give to the panda corys at the bottom. They usually come out around 6pm and glide around the tank and the front.

However, they've been acting strange for the last 6-7 days. I'm getting super worried. I haven't seen snail no.1 since last Monday, and snail no.2 looks kinda lethargic and rarely comes out. I did some digging around my plants yesterday and found him buried in the substrate (haven't been able to find snail no.1). I took him out to check if he's alive, then put him to the front of the tank so I can see how he behaves. He got up and went back to his spot and buried in again. But he seemed to be moving somehow slower than usual, and for some reason, his skin seemed darker than before to me.

I also noticed some discoloration on his shell last week, so I added a calcium tablet and some Dennerle KH+ to raise the KH a bit. I'm attaching a couple of photos of him I managed to snap while he was out.

The tank (125L / 33 gallon) )has been running since December 27th. It cycled for 5 weeks, and after week 6 I started stocking. Parameters have been perfect since, 0 ammonia, 0 nitrite, nitrate has been staying under 20ppm with regular water changes. The only issue (as evident in his shell) has been KH. I've been trying to keep it stable but our tap water is not really KH rich.

As for tankmates, they've been living with espei rasboras, panda corys and otocynclus for the majority of the time I've had them. The only recent change is that I've got a male betta about 10 days ago. I haven't seen him attack any of the fish or the snails, he's just been chilling and exploring. None of the fish seem to have issues with him either, they all behave just as they did before he joined.

I'm looking for any advice someone more experienced with assassins might give. How bad do you think the shell deterioration is, and how else can I help it?
Do you think they're sick because of low KH? Or that the betta might be a factor?
Why are they hiding/missing?

Thank you!

edit:
p.s. the bladder snails have been getting bigger, which never happened before. The assassins always got them before they got a chance to grow

 

[Playsander]

Hi. I have a lot of these guys.

Not sure if low kH could do this as mine is super high.


Assassin snails are notorious for hiding in the substrate. I have a colony myself and rarely see one or 2 out at once. They do breed in captivity very well which could be why you have not seen one for a while (laying eggs) you maybe going to have assasin babies.

The shell looks ok to me.

 

[eicca]

Hi. I have a lot of these guys.

Not sure if low kH could do this as mine is super high.


Assassin snails are notorious for hiding in the substrate. I have a colony myself and rarely see one or 2 out at once. They do breed in captivity very well which could be why you have not seen one for a while (laying eggs) you maybe going to have assasin babies.

The shell looks ok to me.

Thanks! My KH is 0 at times. Water changes help, but I need to do them like twice a week to keep it barely at 2 degrees.

The thing with the shell is that it didn't look like that a couple of weeks ago. The part on the front, closest to the camera, looks somehow too white and almost melted. I'm not very good at snails, so it might just be the way it's growing?

 

[Archerfish]

What I want to know @Playsander, is what delectable beverage is in your avatar glass?