Help very sick fish

[Vienolac]

So I have 4 panda Cory's. And one or them is dying I think... But very very slowly.
It started out he had white stuff on him. And I thought it could be columnaris. But he didn't have lesions or really a lot of the symptoms...
We got some fungal medicine little green tablets idk what they are my gf bought them.
And we used them to try to help with the white stuff. It did and we have been keeping him in a breeding net. Because we don't have another tank.
But now it seems all of his fins have rotted off. Or looks like they have been singed off?...

I personally believe that the fish has no chance of survival due to massive fin loss(picture). He is missions almost all of his side find, and 1/2 of his tail fin.

And I was looking at my other fish. None of them have the white fungus or anything. But one of them does have some spots missing out of his tail... And I don't think the other fish are eating him. Because the last time I had a fish doing that I took the fish out.

Does anyone have any information on what this could be... Or what I can do to prevent doing it again...
My set up
20 gallon
Penguin 200 filter
100 watt heater
Bubble wall
Tanks been setup for 2-3 months
2 Molly's 1 swordtail 4 guppy and 4 panda cory
Did a water change yesterday
Ammo.- .25
Nitrites.- 0
Nitrates.- 10

 

[GNarnian]

I don't think your tank is cycled properly. A fully cycled tank would have 0ammonia in it. This could.be part of your problem, since any amount of ammonia in the tank can harm your fish. I would suggest doing some water changes to get that as close to zero as possible. I would test your tap water to see if there is ammonia in your tap water already.

 

[CichlidSensai]

Re test your water double check those readings.

 

[Vienolac]

Hmm. I tested the tap. And the tank. I can't tell if it's 100% yellow(API master kit). Or if it has a very slight tiny if the green.
But I think it may be 0 safely to say.