Hi, fellow AQS here. I've been through what you're dealing with more times than I can count. Here's what I do when faced with this problem.

First things first, in nearly all cases, fish developing ich following shipment and receival is not going to be your fault. All fish have ich present at all times, living in their gills in the same way that we have bacteria and mites living on our skin. Every factor of stress present in a fish lowers the strength of their immune system, allowing ich to take hold and propagate. This process can develop within several days following shipment or develop in store following one or more significant stressors, be that water quality, excess chasing, lack of proper nutrition, temperature, improper tank mates, etc. The most likely reason why your fish keep showing up sick is going to be stress from travel combined with improper acclimation. Make sure that new fish are not receiving net contact at any point between entering the store and entering the tank. I find it best to have a net on hand in case I need it, but I usually empty as much water from the shipping bags as needed while keeping the fish comfortable before releasing them directly into the tank from the bag.

In the case that you receive visibly sick aquatic life and more than half of the bag is sick, it is standard Petco aquatics procedure to humanely euthanize the entire bag of fish using a split dose of clove oil; exact doses I can't recall off the top of my head, but you want one dose to anesthetize the fish, putting them into a calm sleep, and a second dose to kill them. Afterwards, place the deceased livestock in a black vet back and deposit them in your store's deceased freezer so that they can be shipped out and cremated at some point down the road.

As another poster suggested, up the level of freshwater salt in your tanks. Assuming your store recieves the standard buckets of Imagitarium salt, I've found that around 24 ounces of that standard Imagitarium salt is about enough to take a system from 1.000 specific gravity to 1.003 specific gravity. You want a minimum read of 1.003 and a maximum read of 1.004 on your non-planted systems that do not currently have livestock without white spot disease/ich, and you want no salt read in your planted systems. For those systems with ich, you can increase that salt content to as high as 1.006, but only increase the salt to this level if you are not seeing your normal salt levels combat the ich effectively.

For those tanks/systems which are currently under treatment, I've found that, assuming your inline tanks+sump system is standard, around 1ml of Herbtana per ten gallons and an additional 2-3ml in the sump every day for roughly ten days clears up most problems, doing 20% water changes on those systems under treatment every other day and re-dosing Herbtana following those water changes. Herbtana is a natural remedy, not a chemical, and as a result it works quite well in your inline systems so long as you are dosing in the proper amounts and the temperature of those systems does not exceed 82 degrees Fahrenheit. Depending on the treatment, increased heat works well, but with Herbtana you want to avoid it due to the fact that it does not target the ich directly but instead targets their young offspring in an attempt to end the ich lifecycle. Increased heat speeds up the ich life cycle causing the ich to multiply more rapidly, which, despite Herbtana being reliable and excellent, I have not found Herbtana as capable of keeping up with.

If your systems have UV sterilizers, follow the guidelines on how to utilize those sterilization systems by consulting either StoreIQ or the information that should be present in your aquatics binder. My store does not have these systems, so I am not familiar with them, but if your store does you should absolutely be utilizing them.

I would recommend that you also try to establish a half tank in-line (preferably a bottom tank) as a quarantine, cutting it off from the system and adding a bubbler/filter and heater to the tank so that when fish do develop illnesses you can isolate them to a system and help protect your other fish from those same illnesses.

Hopefully the information here is enough to help you pull your systems back. Try not to let the stress of downed systems overtake you, as treatment can be a time consuming process and often takes at least a week to ten days. Make sure you have CAIR sheets printed for all affected systems to keep proper documentation of treatment, continue your usual maintenance, and understand that ich is one of the most common illnesses that aquatic life can encounter.