My corys have something wrong with them

[Angel Lady]

fin and tail rot. I am starting to get a little worried becaise all they do now is lay at the bottom the water readings are all at 0 except for nitrates and they are 40 ppm. I have all kinds of medicine I just dont know what to give them I have never had to give them anything before this is the first time they have ever had something wrong with them

 

[HeatherSweetness]

Do you have methylene blue? That's excellent for fungal infections. I use it on my corys all the time.