...it died because it was pulled up from deep water.

Fish don't die because they are pulled up from deep water.

Pulling a fish up rapidly from deep water de-pressurizes their swim bladder. De-pressurization causes the air bladder to expand. When the air bladder expands, it can create additional pressure on other internal organs as well as cause the fish to be unable to right itself. This, IF NOT CORRECTED, creates problems with stress, breathing, etc. and can result in mortality.

Fish normally self-regulate their air bladder on a regular basis but this is a slow process. They move from deep water to shallow water (and vice versa) often. If the movement is temporary (i.e., coming up from depth to feed), the fish is not significantly stressed for a long enough period of time to be problematic. If the movement is longer-term, the fish must make the transition more slowly, allowing for the equalization of the swim bladder at the target depth.

The two ways to relieve an expanded swim bladder are to either: (1) Return the fish to depth immediately or (2) Release excess air from the swim bladder ("fizzing"). Often, a fish caught from deep water can swim back to depth if released immediately. If the fish cannot swim back to depth or cannot maintain an upright position on its own, relieving the expansion of the air bladder by properly fizzing a fish enables the fish to quickly return to a neutrally-buoyant state in an upright condition, eliminating THAT source of stress to the fish.

Keep in mind that there are other stressors on a fish that can also be equally fatal. Excessive handling, shock from a change in water temperature, inadequate oxygen and excessive ammonia can all contribute to fish mortality. For example, colder water holds more dissolved oxygen but if you move a fish instantly from very warm to very cold water, the temperature shock can be equally stressful. The key to reducing fish mortality is to eliminate or minimize ALL of the stressors on the fish.

Somebody explain how much to let out. I know in summer, they can die on bottom too(after being fizzed). That "just feel if fish still applies upward pressure on your hand when fizzing" is easier said than done. I understand how to fizz, but how exactly do you get them to neutral bouncy?

no but I had 2 reasons to post this, first to compare to the sharelunker threads. it is a shame that 550 died but had the minnow bucket staff asked just a couple of questions this might not have happened (I am not disparaging the staff either).
second is to maybe make some on here think about what they actually know or can do for a deep caught or gut hooked fish, maybe some will make an attempt to learn proper care if they don't already know how.

not trying to put anyone down because for all anyone knows 550 might have died no matter what so no blame is being placed on anyone.