How long was the tank running with all 7 fish before this started happening?

about a year

I have a feeling that the petsmart people dont test the water accurately. Get a freshwater master test set from aquarium pharmaceuticals. They are under $20 and last a year or more. I also think that petsmart uses the multi-test strips that have been said to e incosisstant and not always accurate. Also, could have been a parasite or just poor water conditions.

How often do you do water changes?

I fo water changes about once a week and I had baught a test shrimp thing and came out fine accept for Ph it was like at 6.8

pH 6.8 is fine. Test strips are sometimes unreliable especially multi test ones. A reagent type test is usually the best.