Welcome to R2R. You will quickly find a lot of small intricacies about saltwater that you can get lost in. Don't worry about the fact that you are in over your head, just focus on the main things of reefing, let me break it down for you.
Main Components of Reefing:
Proper Flow
Proper Water Quality
Stable Parameters
Proper Lighting
Proper Feeding
Proper Tank Size/Stocking Choice
For now focus on the things above, those are the basics. Once you get the things above mastered, you can look into the following, but not until you get the list above taken care of.
Minor Components of Reefing:
Automation
Dosing
Advanced Equipment (Dosers, Skimmers, Reactors, Etc.)
Brand Wars (which brand of salt is best, what type of carbon to use, etc.)
Advanced Chemisty (ULN, High Nutrient Systems, Triton Method, etc.)
Use these two lists to prioritize what you need to study up on first.
I would highly recommend going to the store and seeing if you can return the fish for store credit. The Picasso Trigger is a beautiful fish, but you are not in a good place to keep it.
A Trigger is a predator fish, basically meaning that it needs a specialized diet of fresh seafood from the market or live shrimp added to the tank. These fish don't do well with what most beginners feed like pellets.
When it comes to potentially doing a QT, look at threads by
@Humblefish and follow advice by
@ngoodermuth ,
@4FordFamily ,
@Frtdrmrose7 and others. They are super helpful and always steer you in the right direction all around in saltwater, but especially in disease and treatment.
Right now, your main goal will be to watch the fish for signs of disease. You will also need to start looking for a new home for it, unfortunately. Finally, read as much as you can until your brain hurts about keeping fish, there is a lot of info out there. Once your brain hurts, you have only cracked the surface on the amount of reading you need to do haha.
Carefully read in to what information you find. Look for people on this site that have tanks to back up their info. Watch out for facts vs. opinions. Don't get too caught up in opinions right now on how to do things, stick mainly to the basic facts. Like I said above, don't get into anything advanced just yet, focus on the basics.
Finally, there are no dumb questions. R2R is a friendly community that wants you to have success. As you have already seen, you have a lot of work in regards to your tank, but everyone so far has been nice and helpful, so pleases continue to ask questions.
Best of luck!
EDIT- I would like to add that you haven't necessarily done anything wrong, you were just working on some bad info from the LFS that lead to some bad decisions. Now that you know better, it is now your responsibility to make necessary adjustments to correct the situation. I am just saying not to feel bad for being in your current situation, just make it right for yourself and the inhabitants.