Personally I'd agree with most everyone else saying that it was probably the jack that caused it, it is possible tho that the danio got in the way of the butterfly while it was feeding and got accidently nipped. If ya don't mind me asking do you plan on upgrading in tank size at some point? Because the jack will definitely outgrow that 30g, personally I wouldn't use anything smaller then a 75g as a permanent home for it. 30g is fine for a grow out tank but it will eventually need allot more space.