Be careful with Tea Tree oil Mate. Have a look at this site doesn't look like a good additive for fish.
Tea Tree Oil : Science and Safety | NCCAM
Side effects don't look good, especially
Tea tree oil should not be swallowed. Poisonings, mainly in children, have caused drowsiness, disorientation, rash, and ataxia—a loss of muscle control in the arms and legs causing a lack of balance and coordination