Water Problems

Before we can answer that, how big is the tank, what fish are in there already and is your water soft or hard? The hardness can usually be found somewhere on your water company's website, and you need to make a note of the unit as well as the number. Basically, we don't want to suggest a 2 foot plec for a 30 litre tank!

Shops are often the worst places for advice. Too many of them only train their staff in how to make a sale not in fish care.
Can I ask, what is the stuff the shop recommended to add? And it sounds like you are using test strips, they are notoriously inaccurate. Liquid testers, while still not perfect, are better. You need ammonia and nitrite testers at the minimum.

Water changes are important unless you have heavily planted tanks like Richard W. It is possible that by the time you did the water change that resulted in dead fish, either the water chemistry had changed significantly from your tap water or you kicked up a lot of muck from the gravel or both. Either of these could have harmed the fish. And if you listened to the shop, you would have done a fish-in cycle rather than a fishless one so the fish that died could have been weakened by going through the cycle.
In tanks with no live plants or only a few plants, water changes of at least 25% a week are recommended together with a gravel clean.