Rummynose Tetras

I bought 5 rummynose tetras the other day, and two didn't make it through the first night. There was no obvious reason as to why, so I assume that it's just because these fish are sensative to changes in water chemistry and didn't make it through being put in a new tank. The other three seem to be doing fine. The day after I bought them the color in their tails had become more distinct, and the little color that was around their mouths had gone from the pink it was in the pet store to red. Today (day three), most of their heads have become red back to the eyes, and a little past on one. Since their colors are filling out, does this mean they're happy? I plan on getting more whenever somebody other than Petco has them (theirs looked underfed, with heads bigger than they should've been for the body, but I assume this could have also been intestinal parasites?), but the stores in the town where I live hardly ever have them. This is why I assume that it was water change that killed the two that died, since the city where I got them is on a different water supply, and the fish I've bought here have never died from an unknown cause.

The three I still have do stay close to one another, mostly hiding in the plants I have. Three fish doesn't make up much of a school, so I'm going to stay on the lookout for more. I'd like to end up with maybe 10-12. I imagine they'd be a little more outgoing in a larger school.