When I mix my nutrients, the next day I notice the nutrients have settled to the bottom of the reservoir. How do I stop this from happening?

More water circulation. If you have good air stones already then I would add a water pump to the reservoir not attached to any tubing and let it just circulate the water all the time. This will not only keep the nutrients mixed, but it will help add more oxygen to the water.

Nutrients settled on bottom of reservoir.