A few days ago my QT tank cycled- at least I thought it had,
so I did the big wc and got four more black skirt tetras from a different chain pet store, next town over. I don't go to this store often because it's far, but I've always had healthy fish from them. These new tetras are half the size of my first six, and they are not shy at all. Full colors and hanging around the front of the tank eager for me to feed them on day one. I surmise they have been treated well.
They looked so good I didn't think to test the water that day, but did yesterday. Yikes, a nitrite spike. Small amount of ammonia- less than 0.25 ppm but still discernable. I did a 40%wc last night, and another this morning. Nitrites are not quite as high today, and ammonia is 0 so I think it will be okay as long as I keep close tabs on the tank for now, probably with daily partial wc for a while.