Why would your ph cause ONE leaf to turn yellow?
Yes ph is important as are many other factors. Chill
well if its the lowest leaf(it is), then its the leaf closest to the roots and the leaf that the plant will start pulling nutes from first if a deficiency occurs. your slinging advise like your oldschool and then reference your recent first grow.
the yellowing is definetly a N def, but its quite common with clones because when they are rooting they must use stored nutes to grow roots. do not cut them off as the plant is still pulling valuable nutes from it, when its dry and ready to fall off on its own is when the plants done with it(millions of years of evolution cant go wrong!).
i run my ph with RW as low a 5.0 so no, 5.5 isnt too low at all.