Betta Breathing Heavy at Bottom of Tank - Help!

I’m hoping someone can offer some advice, I’m reading too much on the internet. My daughter has a betta for over a year, everything has been fine. Over the past two weeks, his tank was really dirty so we changed the water (full disclosure, before tonight I didn’t realize you weren’t supposed to do a complete water change...we’ve been removing him into a separate cup with existing water while I empty and clean the tank with water (no chemicals or soaps). (I could have sworn that’s what the lady in petsmart told me!) So there’s always that shock when he goes back into the tank after a water change.

This time is different. He’s been hiding a lot in his little hut, and barely eating. he was also darting a lot in the tank (we have a 2.5 or 3 gal tank). Now he is no longer darting, but a great description I read in another post is twitching or if he swam by his hut or the pineapple we have and grazed he it’s like he freaked out.

My online reading led me to believe he was sick so I bought Betta fix. After three days of using 1/2tsp, his condition has deteriorated to the point of him heavily breathing at the bottom of the tank. my daughter will be devastated if he dies, I need HELP. I have stopped the betta fix for now, I feel it’s doing more harm than good.

We do have a filter, but do not know the temp in the water. I refill it using bottled spring water.

I can go to the store tomorrow and pick up what I need - I just don’t know what he needs to get back on track. I’ll try to upload a pic.

no point flagging me when the picture is blue and I can't see the fish.

What are your water parameters I. E ammonia, nitrite and nitrate? How often were you doing water changes? Does the tank have a filter?

Bettas will become lethargic when water is too cold. I would get a heater and set it at 80F. You need to do weekly water changes of 50-75%. I would do daily changes of 50% for the next 7 days and see if that helps.

You mentioned he was rubbing up against things. Do you see any white spots on him? That would indicate ich. Or, when you shine a light on him, do you see anything that looks like gold? That would mean he has velvet.
Also, can you add a better picture of him? It's really hard to see anything in the one you posted.
As @Sylvan said, we will need to know all your parameters, including the hardness of your water. Do you have a heater? You said you don't know the temperature in the tank, so if you don't have a heater, you should get one. That is extremely important for any tropical fish, especially now if he is sick.
Bottled spring water should not be used in an aquarium because all the minerals have been taken out which allows the ph to swing. If your tap water is not too hard, you could use that. If it is too hard, then you can buy some cichlid sand, put it in a media bag (which you can buy at petco), and put that in the tank. That should add enough minerals to buffer the ph. You would add 1/4 cup for 5 gallons of water.

Sadly most of us here have learned the hard way not to trust petshop employees. I hope he gets better