Cursed Tank/ Acidic PH

[Axys]

So I'm going to preface this with: I work for a chain pet store that sells fish, but I do my best to keep the fish in my care healthy.
At this point, I'm willing to do pretty much anything I can to stop the problems with this tank. We have a sump that's separate from our main systems. It's approximately 75 gallons standing, with a hundred gallons filtering through at any given point. We house our larger fish there, but the PH is so acidic that I just had to move my goldfish out because they were suffering, and we keep having death in it. We've drained the water. Let it cycle. Loaded it up with bacteria, PH blanching chemicals, PH reducer. Let it sit and dry out for a week. Nothing is working, which makes me think it's the pipes.

How do you go about killing the problem in a large systems piping? Should I get in there with a snake to pull anything out of the tubing?
Dismember it and clean individual parts? Or is there a chemical I can use that aren't the PH ones?

Keep in mind, calling someone to come look is out of the question. I've *****ed to my manager about it over and over again and they refuse. They just tell me to drain it again, or shrug their shoulders and say, "oh well."

 

[Aiken Drum]

All the chemicals cant be helping. Trying to mess about with water parameters usually does more harm than good.

A bag of crushed coral in the sump to raise pH is better than trying to do it chemically.

Why are you using pH reducer in already acidic water?

Are you sure you are cycled? The bit about letting it sit dry for a week confuses me as this will kill your cycle. It takes weeks/months to cycle a tank and loading with bacteria at best will speed it up. How have you cycled the tank? Presume fishless if you have taken out the goldfish. Did you dose ammonia?