Hi guys i'm after some urgent help! My 7 month old Murray cod is acting extremley weird today/last night. Sitting on the bottom, gulping alot and even sort of "hiccuping" (will bounce off the bottom).
Started acting weird after i purchased 5 small Yabbies and 4 small feeder fish for him as treat two days ago.
I found 1 half eaten fish after the first day, and 2 yabbies have been taken i think.
I tested waters yesterday and Ammonia was 0ppm, Nitrite was 0ppm but Nitrates were high 80ppm and PH was around 6.2. I did a 50% water change. PH back to 7, Nitrates still in the 40-80ppm.
Water temp is usually at 26, but i dropped it to 24-25 over the past week.

He has been sitting on the bottom, and brushing up against everything, he usually hides in his log and even me walking past the tank he will swim off!
I got my hand in the tank and could touch him and he just moved a little. still swimming but very slowly and bumping into things etc. he just doesn't seem right!

I am worried that he may have gotten a disease from one of the feeders? Ive put the heaters back to 26, and i read about adding salt but saw various amounts on what to put in.
I don't want to try and help him and end up killing him . I added 5 teaspoons of Sea Salt to about 2 litres of warm water and have poured it in.
My tank is 3 ft, and about 120-150Ltrs i think.

Any help, ideas on what it could be/what i can do to help him would be IMMENSLY appreciated.