Thanks for that info. My guess it the bioload is too much for this tank, so the pH cannot stay stable. Whenever you do a WC it goes more towards what your tap water is & then all those fish pea in the water (acid), dropping the pH.
What are your other parameters: ammonia/nitrite/nitrate?
How did you cycle this tank?
For brackish water, you must use marine salt.
As far as your stocking, have you looked into the adult sizes of your fish & the tank size for them as adults?