Fish can suffer from brain damage yes. It's most likely something else
though. What size tank do you have and how many fish, how often
do you replace the water. Do you have a bubbler and a filter.
How often do you do filter maintenance, and how?
Nitrates at 0? Surely a crash has occured. PH 6.8 - okayish, but I'd try
and slowly raise it to 7-8. What is the temp.