Clownfish Dying!

[colioj]

Hey there, I need some help!

So one of my two clownfish started breathing super heavy and wouldn't move from one spot, even to get food. He is now laying on the sandbed still breathing super hard but in a complete different spot of the tank than he normally hangs out in. I think it may be ammonia poisoning but what's weird is that my other clownfish is doing just fine (honestly no signs of distress, he has never done this well). Can someone help me 1) diagnose what is happening, and 2) provide any solutions to make sure that whatever is happening to my first clown doesn't happen to my second? I put these fish in about a week ago after I let my tank completely cycle for three weeks. During those three weeks, ammonia spiked, then went to nearly zero, nitrites and nitrates did the same. Could this fish be sick from my LFS, or is it something to do with my tank? I feel horrible, any help is much appreciated!

Parameters (13.5g tank):

Ph: 7.8
Ammonia: ~0.3
Nitrite: 0
Nitrate: 40
Temp: 76
Salinity: 1.023

 

[schuby]

In case the issue is low oxygen levels, can you put in an air stone? If not, just scooping and pouring water back in can help re-oxygenate your tank.

 

[dylandionysius]

Nitrates and ammonia look really high. Do a big water change