pH is a very important part of aquaponics so here is another blog post about it.
My first two years I was running systems that was full of shells as media, that system really didn't need much pH monitoring at all (or at least hasn't over the 2 years.) I've never seen the pH get down to 7.0 and the only things that cause it to get over 8 are a major algae bloom and a giant water change with well water. That lulled me into a false sense of security about pH.
Now I have my 300 gallon system and have been trying to run the pH around 6.5-6.8 but it seems my reaction times are not always quick enough. I didn't expect the pH to drop from 6.6 to 6.1 after skipping only a day of testing!!!! And with the drop of more than .2 of pH while down in this range, the bacteria can't keep up.
So, my advice is, the further from neutral you are running your system pH, the more closely you need to monitor.
With my big system, a jump from 7.2 up to 7.9 because of algae or a big water change or something never seemed to throw the bacteria off. However, my new 300 gallon system with no shells buffering it, can experience a pH shift of only .4 from 6.5 to 6.1 and suddenly my ammonia is spiking and I'm getting more than a trace of nitrite.
So even small shifts down in the low 6 range can have a big impact on bio-filter function. So, if you are running a more acidic system, keep plenty of buffer materials on hand and monitor closely cause you might not have much time to react before your bio-filter is being harmed.
Best bet is to find a pH where you can keep the system fairly stable without too much crazy effort and try to stick with it because changes in pH usually mean different strains of bio-filter bacteria that are better suited to the new pH must take on the load and multiply so if you are constantly changing your pH drastically, it is kind like having to recycle your system at each change.