Pisces fish, like all fish, excrete waste through a process called ammonotelism. This means they primarily excrete nitrogenous waste in the form of ammonia.

Here's a breakdown of the process:

1. Waste production: Like all living organisms, fish produce waste products from their metabolic processes, including ammonia.

2. Filtration by the kidneys: The kidneys filter waste products from the blood.

3. Excretion through the gills: Ammonia is highly toxic and needs to be quickly eliminated. Fish gills are very efficient at removing ammonia from the bloodstream and excreting it directly into the surrounding water.

4. Some excretion through urine: While most ammonia is excreted through the gills, some is also eliminated through urine. Fish urine is typically very dilute, helping to further dilute the ammonia concentration.

Why ammonia?

* High solubility in water: Ammonia is highly soluble in water, making it easy for fish to excrete it into their aquatic environment.

* Low energy requirement: Excreting ammonia requires less energy than other nitrogenous waste products, such as urea or uric acid.

Note: Some fish species, particularly those living in freshwater environments, may excrete a small amount of urea alongside ammonia. This is due to the lower concentration of ammonia in freshwater, which allows for a more efficient excretion process.