Wow, I am just amazed at the number of protozoan issues that have been popping up lately. That's not a good thing... and makes me wonder what source all of these fish are originating from?
But, with that said, I guess that already is enough info to indicate what I see the problem to be in this situation. The treatment you are in need of is metronidazole laced food. In the case of a tetra I would use a quality tropical flake food such as Tetra.
In the case of the cory catfish I would work with a bottom feeder small pellet and do the same thing... treat the food with metronidazole.
In both cases I would suggest a full 14 day treatment, although I don't know how far gone the tetra is at this point, so I can't make any assurances that the treatment is going to save this fish. A photo of the fish would surely help a lot.
If there are any questions about how to treat the food Byron can help with that as he's just gone through a similar problem and treatment of his own. I would suggest a quarantine tank at this point because I don't know what other fish are all in either of these tanks, so I can't say whether or not this is safe for all of them.
I'm sorry for such a rushed post tonight, as I have been in one other, but I have a very sick macaw that needs attention and don't have much time for anything else at the moment. I will check back in this weekend the first chance I get so I can offer a more detailed post for both cases... so if you wish to list the other fish in each of those tanks I can then let you know if the fish will need to be moved for treatment or not. If there are any questions about the medication to be using and where to order it online please let me know and I can post a link here for you also.
Lastly, I would not be offering these fish medication other than the metronidazole for this problem, so selecting a med that has metronidazole plus other medications in it is not something I suggest. Other medications would only serve to stress the fish by treating for something other than the actual problem and those kinds of medications will not contain enough metronidazole content to do the job that is needed.
Sorry again for my rush, but I need to run for now. I will check in again as soon as I'm able.
Good luck to both of you.