If the pool is not loosing water from the crack it is likely just a
normal concrete expansion crack and the plaster just makes it look
worse. When i moved into my house the old pool had cracks down the
entire shell, on two corners, and the deck had 6 inches of air under it
from 20 years of deck cracks letting florida rains pour in. Most shell
cracks will be caused by the cheap engineering of the deck tied into
the shell (decks settle from uncompacted fill,leaks,etc and pull on the
pool shell)....and concrete just cracks in general.

if you're not leaking i wouldn't worry about it. You can test to see if
the crack actually moves with a crack movement testor. But if you're
not leaking i wouldn't worry about it.

Even new pools get large cracks in the plaster....heck 1/2 the people i
know who had a new pool built have them...It takes a long time (year
even) for concrete to cure. So the plaster goes on too soon and the
concrete naturally has expansion cracks which carries to the plaster
sometimes.

I had a 6 foot crack down the pool bottom, and up two walls. I V'd them
out with an angle grinder, put hydrolic cement in the cracks and had it
replasterd with a plaster/quartz aggregate. I also replaced the deck.
Its been 5 years so far and no cracks have come back out.

Average lifespan of the plaster is probably 10+ years depending on chem
maintainenance. If you did have a leak you could always have those
fiberglass come in and coat your shell but that stuff in my opinion is
overpriced..fiberglass in water gets blisters, delaminates,etc. Luckily
we're in florida plastering is cheap.

- shawn