Here are a few things I always have around that help me take care of my plants.
I always have either apple cider vinegar or regular white vinegar around. It can be used as a trap for fungus gnats expecially when it's mixed with liquid soap.
Alcohol is great for getting rid of spider mites.
I have several spray bottles on hand at all times. I have one filled with regular water that I use to mist my plants. Another one is filled with a liquid soap and water mixture, which is great for gnats as well as spider mites. Then I have one that is filled with a water and peroxide mixture, for gnats as well.
I collect rainwater anytime it rains. I love watering my plants with it.
I water my plants with Epson salt once a month and they love it. It's also good to mix with soil and you can sprinkle it on the top of your plant's soil.
These are the things I always have that keep my plants healthy and pest free.