If water leaks, it's going to want to leave the tank, not stay in it. That said:
Most commercial stains are going to be poisonous. Natural ones, like something you might cook up from walnuts, will be poisonous as well. Many stains change color over time, so what looks good today may not in a few years.
You might be able to tint your epoxy, following it up with some clear coats. Or just mix a brown or grey color into the epoxy and forget about the wood grain.