Habrosus fin rot?

[Elfgirl]

I’m pretty new to the hobby.  I have two tanks.  One with guppies in a 10 gallon and another that’s new that’s a 20 gallon long. They are both planted.  I purchased 4 salt and pepper corydoras on 6/18.  I added them to my guppy 10 gallon tank.  I seemed to me the guppies were too active for the corydoras especially when it came time to feed them.  One corydora died and another one looked unwell as well within about two weeks.  I was prepping my 20 gallon long for them so I moved the remaining three to the new tank.  They are the only inhabitants with the exception of pond snails.  The cory that looked unwell died within a few days.  The remaining two corydoras have been in the 20 gallon since.  I just noticed one of them not as active recently and their dorsal fin seems to be down and smaller.  Barbels are mostly gone on both fish as well.  I just started treatment of maracyn yesterday.  I just love my little fish.  Any help is appreciated!
Water Parameters:

• pH 6.8
• Nitrates 10
• Hardness 300
• Nitrite 0
• Ammonia 0
• KH/Buffer 60
• Water Temperature 75

Edited August 25, 2022 by Elfgirl

[Chick-In-Of-TheSea]

When you cannot see obvious symptoms, it could be internal. Possibly parasites. Missing barbels could be caused by a substrate that is too sharp, or possibly fin rot as you suggested. One super helpful tip that I learned is quarantining new fish to ensure they do not infect fish you already have. Check out this video on how to make a simple quarantine tote.

Here is another helpful blog post.

https://www.aquariumcoop.com/blogs/aquarium/how-to-treat-sick-aquarium-fish

Also, if you have no idea what you're dealing with (what illness it is), aquarium salt covers pretty much everything.  https://www.aquariumcoop.com/blogs/aquarium/aquarium-salt-for-sick-fish

[Elfgirl]

Thank you.  I’ll try the salt today.  Now I have a quarantine tank as I unfortunately learned the hard way.  I currently have some green neon tetras there for the last three weeks.  I guess they will remain there until I am sure my Cory’s are healthy.

[Chick-In-Of-TheSea]

I'm going to tag @Colu also because Colu is familiar with a lot of fish diseases. Let's see if we can get any additional input, just in case.

On 8/25/2022 at 10:02 AM, Elfgirl said:

Thank you.  I’ll try the salt today.  Now I have a quarantine tank as I unfortunately learned the hard way.  I currently have some green neon tetras there for the last three weeks.  I guess they will remain there until I am sure my Cory’s are healthy.

I went many years in the hobby without a quarantine tank.  So did my brother and Dad who also had fish tanks. Most of the time we were lucky, but sometimes not.  That was before the YouTube days.  I have learned a lot along the way.

I've got a fishy in quarantine as well right now.

Also, I forgot to say - welcome to the hobby and to the forum!  

Edited August 25, 2022 by Chick-In-Of-TheSea

[Colu]

What I would do is finish the full course of maracyn as the barbels loss can be caused by a bacterial infection and as you have lost fish over several  weeks I would be suspicious of internal parasites what I would do is follow up with a course of paracleanse in food @Elfgirl

Edited August 25, 2022 by Colu