Albino Cory is not moving

I had an albino Cory alone for about 2 weeks and then got three more today because he seemed sad and wasn't moving. I didn't originally know they needed more than one. He still is way less active than the other three and I'm worried that he will die. He has a slight red thing on his tail but it doesn't look like anything bad but I don't know what to do.
This is the tail

 

It's hard to say without knowing the details of your tank. A healthy cory will move around and act normal even if it is by itself, so it is probably ill and not just lonely.

Can you fill in the emergency template?

 

It's hard to say without knowing the details of your tank. A healthy cory will move around and act normal even if it is by itself, so it is probably ill and not just lonely.

Can you fill in the emergency template?

I change the water at least weekly if not more. Heavily planted tank. Cycled beforehand. 0 nitrite. Haven't checked the nitrate super recently but I assume very close to zero because of the amount of plants I have. I have other fish that are acting normally. A couple of guppies and tetras and cherry shrimp. Temp is 75. Ph is probably not ideal because it's around 7.8

I change the water at least weekly if not more. Heavily planted tank. Cycled beforehand. 0 nitrite. Haven't checked the nitrate super recently but I assume very close to zero because of the amount of plants I have. I have other fish that are acting normally. A couple of guppies and tetras and cherry shrimp. Temp is 75. Ph is probably not ideal because it's around 7.8

I don't know if his behavior originally was the same because I didn't know the usual Cory cat behavior. I assume his behavior was around the same when I got him because I can't remember a time where he acted like the new ones

 

The substrate could be culprit.
Can you place a tank-pic

 

The substrate could be culprit.
Can you place a tank-pic

We can see the substrate in the tail pic and the substrate looks bumpy