if a parent has mental issues and the parents are seperating who gets custody

When parents separate, courts will generally try to keep children in contact with both parents and divide custody in accordance with the best interests of the child.  Having a mental health issue alone would not disqualify a parent from custody of the children, and even someone who is symptomatic may be able to spend time with the children. Courts may order custody evaluations or require evidence of participation in treatment.  One parent cannot decide that the other has "mental issues" that affect custody -- there must be some evidence to support it. That evidence might be behaviors that support an evaluation.  It all depends on the circumstances.