Pygmy Cory Falling ill

[DeviatorZa]

Hi all. I need advice. I have a pygmy Cory that seems to have fallen ill. It's lethargic - not sure if it's fin rot / bacterial, but I've tried treating with half dose of Seachem Polyguard (as instructed), bearing in mind that there are shrimp and this is not ideal for them. I've now removed this cory and put it in a quarantine tank. What I'm trying to do, however, is remove the underlying cause. In terms of water quality and parameters:

• pH: 6.8
• Nitrates: < 10
• Hardness: Water is on the soft side, but within allotted range on strips
• Nitrite: 0ppm
• Ammonia: 0ppm
• Water Temperature: 25 degrees Celsius

I'm not sure of buffering capacity, but my ph doesn't seem to swing all that much before / after water changes. 
Aquarium setup: 8l/m (4x2) 2 sponge filters on a 60l/15gallon tank,
Stocking:

• 8 Eyespot Rasboras,
• 10 pygmy corydoras (8 of which are juvenile)
• 40 Neocaridina shrimp (Various ages)
• 1 Female Guppy
• 2x Nerite snails
• Few bladder and other pond snails

Water change schedule: weekly 25% water change

I suspect it might be in the substrate, or below it that might have gotten bad. I'm not sure. I'm using Prodibio Aquasoil, but do have some of the old gravel that was washed below it for some height. Is this where I'm going wrong? Any suggestions would be appreciated.

Edited October 13, 2020 by DeviatorZa
Clarity

[Ben Ellison]

I'd lean towards parasite. How long have you had them? Where they treated for parasites when you got them?

[DeviatorZa]

Quote

I'd lean towards parasite. How long have you had them? Where they treated for parasites when you got them?

Hi @Ben Ellison. I've not done the parasite treatment. Is it possibly something internal parasite leading to a weak immune system compounding other problems? I've had this one for 6* months now. It's one of the only two survivors out of 8. They all inexplicably died one after another despite all I tried. Whatever it was seemed to have zero effect on the other fish. The 8 new tiny ones were introduced after two weeks in their own quarantine tank two weeks ago. They are doing fine. I guess I need to start adding para cleanse or something similar if I can find it out here to the quarantine regime.

Edit: I also only noticed the tail looking like that today. Hopefully there's still a way to save this fish. Thanks for taking the time though.

Edited October 14, 2020 by DeviatorZa
Clarity of response

[DeviatorZa]

Update on the cory. It unfortunately passed away during the night in the quarantine tank. I believe @Ben Ellison is probably correct. I'll have to add parasite treatment to the initial quarantine period.