@Zhao yun There is a condition known as 'hole in the head disease', believed to be an infection with something called Hexamita.This infection can also be internal, which is hard to diagnose. It is treatable, but have never seen a case like this and I suspect this fish will not recover.
Edit: I have just checked, Hexamita is a flagellated protozoan.
@FoxBat this is not C. ocellaris, or any of the similar-looking species like C. monoculus or C. kelberi. To my knowledge, while interpretives exist, C. ocellaris is not present in the collection. Also, not a platinum for sure.
To me, this looks like any of the following, although a full-body shot will be required to identify the species:
- Cichla piquiti (Azul Peacock Bass)
- Cichla orinocensis (Orinoco Peacock Bass), this is the most likely to me.
- Cichla melaniae (Xingu Peacock Bass), this is a close second.
- Cichla intermedia (Royal Peacock Bass)
@Zhao yun the animal is suffering from Head and Lateral Line Erosion, or simply HLLE or HITHE (Hole-in-the-Head Erosion). This is often a result of poor water quality, parasites, heavy metal/mineral content in the water, or a combination of some or all of the above.