It sounds to me that the fish are probably still acclimating and went through some sort of shock. Doesn't sound like temperature is the culprit, but it could be pH shock since it doesn't sound like you directly acclimated them with the tank water.
From personal experience, I've found that drip acclimation using an airline hose works wonders for all sorts of fish and takes almost no work if you have the necessary equipment.
- White gallon size bucket
- Sufficient length of airline hose
- 1/2gph or 1gph drip-valves sold at Lowes or other hardware/gardening stores (they're usually found in the rain, plumbing, or gardening sections and are super cheap <$1)
- Clip or suction cups to anchor airline hose
Just fill the bucket with just enough water to cover the fish and then run the drip for as long as it takes to fill the bucket up 2x (empty the bucket to 1/4 after the first fill-up). If you know how much of a pH difference there is between your water and the water at the fish store, you can figure out just how much drip acclimation you need to do. A pH difference of 1 will require about 10x dilution (so if you had 1cup of fish store water, you need to add 9 cups of aquarium water).