why did my fish die within 4hours?

Brandie is right, no way to know without symptoms. Can I suggest you be ready to find a larger home for your fish soon? Alligator gar easily hit 8 feet long and over 300 lbs. No doubt yours is already suffering badly in a 55 gallon tank. They are also a protected species and illegal to own without a special research permit.

MM

I don't think anyone has hit on the exact answer yet.

How often do you clean/do water changes in the gar's tank? And when? Anything the size of these fish that eats other fish is going put out a lot of ammonia. If you've had him a while, you've probably got the tank cycled well enough for him. But those bacteria are converting ammonia to nitrite then to nitrate. Have you tested your nitrate levels? While the gar's probably adjusting to the elevated nitrates from his previous meal, the feeders are living in (relatively) clean water in your dealer's tank. You bring them home, dump them in and "Ugh! Nasty water!" (at least in the opinion of the feeders). Not having been acclimated to high nitrates, they drop off quickly.

Now you're dealing with massive die-off, producing an ammonia spike - no surprise, this starts to affect your gar (ammonia poisoning) which goes away when you put him in another tank (in which he hasn't been defecating and no fish have died).

You can confirm this easily enough by doing a few ammonia and nitrate tests. IMO, do a water change of about 1/3 to 1/2 the tank's volume the day before/the morning of adding feeders. Depending on the size of the gar and how often you feed, this could be two - three times a week. But if the water quality is good when you add the feeders and you take the time to acclimate them to your tank they should last longer. You may need to use a tank divider during acclimation to partition off enough room to float the bag - I'd try to get an opaque one if possible - there's nothing like a big mouthful of teeth biting at your bag to make a feeder fish feel like "home" and scare even more "ammonia producing product" out of them!

Wow, you got three crazy's in a row. Anyways, the real answer to your question is, you may have old tank syndrome if you arent cleaning your substrate enough. You need to check your water paramters, make sure nothing is out of line. If that checks out, I would do a standard tank cleaning and water change anyway before you put him back in, b/c feeder fish often carry disease and/or parasites. So what it boils down to is, the feeder fish were diseased and died, you have old tank syndrome, or you possibly just shocked your feeders when you put them into his tank (if they were'nt acclimated, that is.) No way to really tell, unless your gar gets sick and shows some symptoms.

It was either the fish drowned, the shock from different water temperatures killed it, it died from ammonia, or because the system wasn't setup at least one day before.

was the water pre laid out 4 i hour if not there body goes int shock

they most likely had new tank syndrome !!!!

because he drowned