The fish has a bacterial infection eating its tail. The black patching on the body is normally bruising caused by chemical burns from ammonia, nitrite, low pH, chlorine, fish medications or plant fertilisers.
Does the tank have a filter?
How often do you clean the filter and how do you clean it?
How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before it goes into the tank?
Do you have buckets used purely for the fish tank or do you use any bucket in the house?
Have you tested the water for ammonia, nitrite, nitrate, pH and GH?
If yes, what were the results in numbers?
Make sure any new water is free of chlorine/ chloramine before it is added to the aquarium.
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I would do a 75% water change and gravel clean the substrate and then add 2 heaped tablespoons of rock salt, sea salt or swimming pool salt for every 20 litres of tank water. Keep the salt in the water for 2 weeks and see how it goes. If the problem gets worse during the next week, then get a broad spectrum fish medication and treat the tank. But try the salt first.
The black marks will take at least a few weeks to go away and normally takes a month or more.
The tail is the main issue and you want the red inflamed area to go back to normal colour and look normal (not puffed up/ inflamed). If the infection is treated successfully, the tail will grow back within a month or so.
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To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating with chemicals or it will absorb the medication and stop it working. Carbon will not absorb salt so can be left in the filter when using salt.
Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water.
