What can this be?

[Barry Tetra]

They dont have numbers on the strips it's a colour chart

Then, i think its the fin nipper fish or maybe zebra loaches i have a problem where they the loaches suck others skin to death. The red spots on body too

 

[Barry Tetra]

They dont have numbers on the strips it's a colour chart

You might have to remove the loaches.

 

[Waterstrider]

I don’t think you provided enough information to know much. I’d at least want the number of each fish and the size of the tank. It doesn’t look like nipping to me but I don’t think anyone can know without more information

 

[PheonixKingZ]

How big is the tank? How long have you had the tank running? What are your full water parameters? Which did you put in first, the corys, or the loaches?

Edit: I just realized that you didn't have sand in your tank. Sand is very much so recommended for cory cats.

 

[Waterstrider]

How big is the tank? How long have you had the tank running? What are your full water parameters? Which did you put in first, the corys, or the loaches?

Edit: I just realized that you didn't have sand in your tank. Sand is very much so recommended for cory cats.

And loaches

 

[Byron]

Diagnosing disease or other problems is not always easy, and members really do need to know all they can in order to try and advise. Numbers for tests (ammonia, nitrite, nitrate) are essential, or a photo of the colour chart and strip so we can see. We may have different interpretations. Also, parameters of the water are important to know, meaning GH, pH and temperature.

I have often seen dead fish appear like the cory in the photo, but that does not tell us much as there are several possible sources and a dead fish is likely to have red marks and disintegrating fins which may or may not be the initial problem.

One thing though, you should never have loaches and cories together. And loaches are social fish that must have a group of at least five of the species or they will be severely stressed. It is certainly not impossible that the lone Zebra Loach (I am assuming from the wording that there is one) killed this cory overnight or during the day.

Colin's advice in post #7 is about as good as anyone can give without more data.

 

[Freshwater Sucker Fish]

The fish is gone now

It's okay there Is a fish heaven. Where all pet fish swim in the Clouds ^-^