I've been a pool owner for about 40 yrs now. I've had marcite and fiberglass pool finishes. We bought a new house 5 yrs ago with an ugly pool. Last April (2015), we had it resurfaced with hydrazzo. In April I noticed this mark on the wall in the corner that I was able to brush off. Thought it may have been a creature that got into the pool during the winter when it was covered. This weekend I noticed it was back and that it was actually a crack in the hydrazzo. It's about 18" long.
The pool is 18 x 36 rectangle, depth is 3'-5'. I have a variable speed pump and an electric heater. I live in Fl so the pool was covered for only 3 months and I kept up with testing and adding chemicals. The pool was built in 1987. I have all my pool chemical testing papers and they are all good since it was refinished with the exception of chlorine and Ph being out of whack slightly after a party or heavy use. I brush and vacuum the pool weekly.
My question is what could have caused this crack?