if you do everything right fish will live for over10 years. If one or two fish died then it's may be has been a sick fish to start with or didn't handle transportation/rehoming stress to well. Looks like it's not the case and in fact it's either you doing something wrong or fish prefers to die than live in your right size tank with a filter air and 1 plant. I think it's first. Give us more details and someone will point you in right direction