noticed this after feeding my fish and medication

[Calc]

Hi guys

Recently there is a blue green algae blowout in my tank,
I noticed one of my lal cheng is staying at the water surface all the time,
after feeding them with market shrimp and erythromycin for the algae problem.
Other 4 lal cheng is very active and still eating well.. wondering what is going on? and what could have caused this?
what can i do to cure this little guy? I just did 50% water change and added conditioner with salt.
Ph: 7
nitrate:0
nitrite:0
Amonia:0

 

[Owain4]

nitrate:0 you got a problem. There should never be 0 nitrates, i suggest getting a better test kit.

There is always nitrates in tanks as they are the result of the nitrogen cycle that breaks down waste. what do you use to test your water?

 

[Calc]

Is there a chance the fish is suffering from constinpation?

 

[Calc]

I am using nutrafin test kit
Here is a updated pic, and the lower end between gill and stomach part is bloated.

 

[MyGiants]

Blue Green algae blooms in High Nutrient low moving water. Best not to treat just siphon it out clean the filter cut down on feeding increase water flow.

 

[Calc]

but what is going on with my fish?...any idea?