Why did one species of fish die suddenly overnight? The rest are fine.

We have guppies (12) and rummy noses tetras (6). All the fish were fine and active last night before bed and when we woke up, ALL the rummy nose tetras were dead. Guppies are fine.

We didn’t change the water, the tetras noses were red (indicating decent water quality). 2 of those tetras have been with us for 11 months and the 4 for about 10 days. They were totally fine and active yesterday evening.

The only thing that changed was we now also use guppy pellets (which really is the same, just much smaller to fit their mouths). We started this 10 days ago (we bought the food same time as when we bought the 4 new tetras). The fish seemed fine with this food, there was no slow decline or anything- in fact, the 4 seemed better (from pale to red and active).

What could cause this? It’s just weird that all the rummies died suddenly overnight and the guppies are completely fine. The guppies are also much smaller and by far not as agile or as strong as the tetras so I can’t imagine them killing the rummies.