I've had a small cold water tank for about 3 years now.. nothing serious. Been keeping Platys and later on added some Danios (1 zebra 1 leopard).. we have always had quite a good success rate. Having lost a Platy and down to just 1 and the Danios we bout another Platy at the weekend and decided to add a couple minnows which the aquarium centre said would be fine. Within a couple of hours of adding them one if the minnows had been attacked (bitten tail) and wasn't looking too good.. the following day it had disappeared! Would the Danios have eaten it??? I'm worried about the other minnow as it's on its own now.. staying out of the way so it seems ok but I don't want the same to happen again. I'd obviously like another minnow but really wary now. The platy has settled in nicely.
The other fish may have eaten it, but only after it died of whatever cause. Zebra danios can be quite aggressive and nippy with other fish unless they are kept in large groups of a dozen or so and minnows are shoaling fish too so need to be in groups of at least 6. However, this is dependant on your tank being big enough. For Zebra danios at least a 3ft long tank is recommended due to their fast swimming pace and activity and for minnows at least 60 litres, 2ft long for a group.