Some great ideas on plants to repel those nasty bugs without using chemicals that are potentually harmful to your koi. 
 
When it comes to the inevitable face-off between you and bugs, keeping them away from your immediate vicinity is probably top priority as they thrive during the warmer months. While there are always options like pesticides and bug traps, it certainly doesn’t hurt to explore potential natural solutions before trying harsher chemicals (even though they can be necessary with serious infestations).
 
10 Plants That Repel Bugs - Herbs, Shrubs, Flowers Insects Hate (prevention.com)