Gnats suck and Mites are terrible. Even when I get a small infestation my plant shows the stress pretty quickly.
Get some water, mix with a few drops of dish soap. You can add tobacco for an even more effective solution, just soak in water. I've also heard of pepper being added.
Get up under those leaves where the mites are and spray down at least twice a day. You can go back and cleanse the plant with just water afterwards.
It shouldn't take too long for them to go away. If the plant is stressed it will take a bit longer for it to snap back.
Gnats like moist soil, unfortunately so do plants.. if you can restrict access to the soil and let the top dry out a little it will help.
And make sure that you eliminate all possible sources of feeding and breeding nearby. This includes trashcans, garbage disposal and drains, food left out especially breads fruits and vegetables.
A great tool is a trap, set in the same vicinity as the plant but not right next to it. Get a glass, fill it 1/4 inch with apple cider vinegar.
Construct a simple paper funnel that sit s on top of the glass, make sure when the funnel sits in the glass there are no gaps on the sides. Use as much tape as necessary. The gnats should climb into the glass never to come out again.
