How can i reduce stress level?

I do not understand how a fish can get srtessed?
I have specially bought a medicine called "stress Enzyme" to reduce the stress level.
is there anything else i can do to reduce the stress.
one thing i have noticed , when i look at my guppy she get frightend and starts to swim here and there but i do not understand how come in pet sop the duppies are heppy where lots of people lokk at them.
i hope u get my point

because shop fish are used to people walking by them, it also maybe to do with those secret fishy stress balls they give the fish

If you change the water or anything in their environment, they can get stressed. Does your fish require a friend? That could be troubling it too. Consider all the things you would do for yourself and transfer that to your fish. Or, call the pet stor you bought it from to see what it requires to have a healthy life.

Guppies are communal fish and need to be in groups..If you just have one I suggest you get at least 4 or 5 more. They feel safer in groups...Fish do get stressed. Is there enough oxygen in the tank and is the water clean?
May I suggest you re ask the question but include the word 'fish' as this has ended up in the human respiratory column.