It is not quite as simple as a fish tail "growing back" from the base. Here is why:
Regeneration varies by species: While some fish species can regenerate lost fins and even parts of their tails, it is not the same as a lizard regrowing its tail.
Tail structure: The base of the fish tail is where the vertebrae connect, and these bones are not typically regenerated.
Regeneration limitations: Fish regeneration is often limited to specific tissues, such as fin rays and muscle. The extent varies greatly between species.
What happens when a fish loses its tail?
Injury type: If the injury is severe and involves damage to the vertebrae, the fish will not be able to regenerate the lost portion.
Species: Some fish, like zebrafish, have remarkable regenerative abilities and can regrow a significant portion of their tails. Other species have very limited capacity.
Healing: Even if regeneration is possible, the new tail will likely differ in shape and size and may lack some original functions, such as powerful movement.
In conclusion: While some fish can regenerate parts of their tails, complete regrowth from the base is uncommon. The outcome depends on the severity of the injury, the species, and the individual fish's capabilities.