Welcome to the forum!
Sorry it seems you've had such a rough start.
The fish are producing waste currently, in the form of ammonia. This ammonia is toxic to fish and can kill them. Ammonia is removed in a cycled tank by beneficial bacteria that converts it to nitrite (toxic) and then nitrate (non-toxic in low quantities).
Since your tank is currently uncycled, the ammonia will be free in the water until you remove it. A 50% water change will lower the amount of ammonia and keep your fish healthy and alive.
Don't trust the people at the pet store. If you get your water tested there, get the NUMBERS, otherwise they have no idea what they're doing and will tell you it's all fine, when in fact it isn't.
Prime by Seachem is a great water conditioner that will temporarily detoxify the ammonia in your tank while you are cycling.
API Liquid Master Test Kit will be invaluable to you during the cycle to track your ammonia, nitrite, and nitrate. You can purchase it in-stores or on Amazon.
Water changes are to be done weekly at about half the volume of the tank. BUT, until you're cycled, you should be doing 50% water changes daily to keep your fish from being poisoned by the ammonia in the tank.
The cycle can take quite awhile. Tetra Safe Start may help jump your cycle. If you have any friends with tanks, you can ask to borrow a bit of their filter media or a decoration for the beneficial bacteria that would be on it.
Just to be clear, the tank you have is a 25 gallon stocked with 1 female betta fish, and 6 glow light tetras?
I also strongly recommend not getting anymore fish until you have completed your cycle. It will only put them in danger and make your cycle take longer.