Is this Clarkii Clown sick

[MadreeferNJ]

Gave my mated pair of Clarkii clowns away and they reside in a 20g tank by themselves. The person added a few snails and crabs which died and the female clown is not doing well. It had white stringy poop and leans to the side a few days now but will swim at times. I see no physical signs of injury. The only thing off was nitrite was high but using Prime that came down. All other tests show in range so wondering what can be the cause. He wants to give it back but if its sick don't want back in my tank.
Tank is up about a month cycled with live rock from my tank and live bacteria. Any help appreciated.

 

[vetteguy53081]

Gave my mated pair of Clarkii clowns away and they reside in a 20g tank by themselves. The person added a few snails and crabs which died and the female clown is not doing well. It had white stringy poop and leans to the side a few days now but will swim at times. I see no physical signs of injury. The only thing off was nitrite was high but using Prime that came down. All other tests show in range so wondering what can be the cause. He wants to give it back but if its sick don't want back in my tank.
Tank is up about a month cycled with live rock from my tank and live bacteria. Any help appreciated.

Inverts dying is a sign of water quality issue
How are you testing water?
Please post a video of tank in whole

 

[MadreeferNJ]

its not my tank but a friend just getting into the hobby. I agree inverts dead is water quality. He assures me all params in range but just found out he's using API kits and I personally find them inaccurate. He has done numerous water changes and told him to stop that.

 

[MadreeferNJ]

The guy added some parasite med and an hour later clown swimming around. I assume the inverts caused this problem.