I agree. Don't try chasing the 'ideal' pH, particularly if you have fry. Fish can usually adapt to a less than ideal pH as long as it is stable. Fry are very sensitive to swings in water conditions. Trying to force the pH up or down with chemicals, and without an understanding of the mechanisms behind it, is most likely going to end in disaster.
One thing I did notice from your original post is when you said:
mircoworms and sometimes brine shrimp hatchlings
So does this mean you still feed your fry predominately on microworms if you are only sometimes feeding BBS? At four weeks your fry should be getting more than just microworms. It could very well be that your fry are simply not receiving adequate nutrition to grow and thrive.
I would not bother trying to cycle a grow-out tank. You should be doing enough water changes as part of your regular maintenance to remove any ammonia that builds up.