Are parasites common in fish?

I wouldn't say that they are common, but they are the unfortunate part of keeping any animal that has been bred elsewhere, shipped (where their immunity really drops and they can become infected easily), and then placed into their final aquarium once purchased from a store that is also holding them in a community styled tank.

I would always suggest asking what treatment any fish has been given once it has arrived at the fish store, and trying to watch for it over a period of time > 5 minutes to make sure that it's acting normal (especially once it comes to eating). As always, I HIGHLY recommend placing the fish into a quarantine tank for a few weeks just to make sure that the fish doesn't show any other illnesses that it may not have displayed at the fish store.

The most common way a parasite infection takes off is when a new fish is added into an established system. So it is always better to quarentine the new ones for 4 to 6 weeks in a separate system.

Wild caught fishes are prone to carry parasites, it is estimated that around 20% of the wild caught fishes may be prone to parasitic infections.

My friend had given me her fishes as she was relocating abroad but to my utter disbelief, I found that it was parasite infested and it went on to infest my fishes too.

I can't say I often see parasites in fish to be honest so I wouldn't say that they are common but I would say that it is something that can happen.

It is quite shocking though that you noticed one in the pet store and even though you told them, the fish was still there. I too would be worried about using that pet store to buy fish especially if they do not take things like that seriously.