Could be the pH but more honestly I think it's the fishload. The high amounts of organic wastes from all those fish will also likely be driving down your pH by producing organic acids, especially as there's some messy eters in there.
Have you tried a ntrate test to see what's in there , and it's probably worth doing ammonia too just to be safe. How much live rock do you have, what substrate, how deep. How much stuff are you pulling out of the skimmer - with those boys it should be a lot , a cupful every couple of days. Sorry, I'm not familiar with DAS systems - is that a wet dry or similar.
I would go for a bit more water changes my self. 10 or 14 gallons weekly would be more normal, especially with those 2 puffers.