Apistogramma Cacatuoides issues.

[TicketedBullet]

One male Cockatoo sitting near bottom of tank under cover. He is not eating, mouth is slightly open and gills moving rapidly. Colors still look fine. 

It's a 29 gallon heavily planted tank with aquasoil and sand substrate seperated by rocks and driftwood. Using an altered HOB filter with prefilter sponge. I tested the water when i noticed the Cockatoo not eating which is incredibly irregular, and the PH had dropped to 6(or below). I did a 50% water change and the current readings are below. I live in Florida so tap's PH is typically 7.5 and up. Tankmates are 3 GBRs, 3 Guppies, 3 banded Kulhi Loaches, 2 Ghost Shrimp, and an undeterminable number of snails. 

Temp: 79

PH: 6.8

KH: 0-40

GH: 300

Nitrite: 0

Nitrate: 0-20

Can't get a good picture/video as he won't come out from cover. I don't see anything in his mouth or any obvious injuries/ visible parasites. 

[TicketedBullet]

I should add that all other fish in the tank are fine. He was the unchallenged boss of the tank up to this point. 

[Colu]

It could be shocked with sudden drop in pH  I would add a buffer I put crush oyster shells in a filter bag in my filter they could be something else going on  try to get a picture

Edited June 10 by Colu

[TicketedBullet]

*Update*

added cuttle bone to the tank last night, checked parameters today and the KH is still reading 0. The apisto didn't make it, I checked his mouth and gills but didn't find anything out of the ordinary. All 3 adult guppies and 1 fry are all fine, if the issue was PH shouldn't they have been affected? I should also add the apisto was in my tank for roughly 3 weeks and arrived full grown, so age could have been a factor. 

[Colu]

It difficult to not  what killed it  i would monitor other fish make sure they are eating and behaving normally

[Colu]

 kH will slowly increase  test in a couple of days

Edited December 21, 2020 by Colu