My royal gramma was like that ,swam upside down out of the bag and very lethargic for a few days. In the end, he recovered and was fully normal in a week. However, if the LFS has a return policy for credit or cash...I may consider using that now. Bringing a sick fish back to health is not easy or for beginners unless you have no choice.
Might want to make sure your next fish are 'easy', like the clown fish. Get a cardinal, a gramma, maybe a fire fish or goby. They are much more tolerant of aquarium life, take less food, produce less waste. Look up any online vendor for 'beginner' fish. No shame in beginner...they may not be the sexiest, but they are beginner because they are hardy and forgiving of our mistakes. And to be honest, I still love my beginner fish.
I wouldn't worry about starting any refugium if you haven't yet. It's just one more thing to do, one more expense (light) and water changes will be fine for now for nutrient control. Learn to use the skimmer and how to maximize removal first. Learn how your tank responds, nothing better than testing, water changes, tweaking a skimmer and adjusting food quantity and type to see what's what at first.