Fantail lethargic

[helptoall]

Hi ,ive rescued him 2 weeks ago and he is currently in quarantine of 60L with filter and oxigen. parameters are 0,0,10. During all this time he was happy and active but during the last 2 das he seems lethargic , spending alot of time on the bottom or just floating in the middle of the tank. He is eating and hes poop seems healthy, and i dont see any external signs of something wrong
What do you think?

 

[Colin_T]

What's the first thing I ask for when people ask for help?
Picture please

 

[helptoall]

hes in a box which isnt tranparent so you wont see anything with a pic, but ive examine him, he dosent have any external signs

 

[Colin_T]

Do a water change and add some salt. Use 2-4 heaped tablespoon of salt per 20 litres (5 gallons).

Feed it some plant matter like duckweed.