Spider mites:

Spider mites are by far the worst pest to have, because they are the hardest to kill and can literally kill any plant. They are by far more active in warmer climates than cold ones, they also suck sup from the leaves leaving tiny white spots on the leaves from where they suck on. The damage they do is just like thrips, but with spider mites they pierce the surface of the leaves and can show tiny holesor white speckling damage, unlike with thirps they eat the surface of the chlorophyll. You can tell when you have infestation of spider mites , because you will have distorted growth, shortened internodes and petioles.
When you have bad infestation you will see webbing with larva in them. You get spider mites from either store bought soil mostly when your soil is not sterilized. Most brands do not state weather or not they are, chances are if you buy cheap soil, you have a less likely chance of the soil being sterile.Also, bringing outdoor plants indoors, or getting plants from someone else who was infected with spidermites.
Spider mites live on the plant itself and are under the leaves most of the time and are usually are so tiny you can't see them with the naked eye.

In order to get rid of spider mites one of the best ways to start to get rid of them is to use NO-Pest strips.NO-Pest strips work well and should always be used with other methods. Neem oil helps get rid of them as well. Also by raising the humidity in your grow room a lot will kill the spider mites off, by them absorbing to much humidity and will burst. You can use soap solution like Safer Insecticidal Soap to get rid of most aphid problems.

Pyrethrum should only be used in extreme circumstances directly on plants, It starts to breaks down around a week and is easily washed away with clear water or. If your plants are in flowering and you have spider mites, using safer chemicals is your best bet. The tobacco and pepper soap solution works well and should be used daily, spraying on the underside and top leaves and later the whole plant. Since spider mites are very residual to common products, you have to find or try any number of ones untill you get rid of them. make absolutly sure you retreat as stated, other wise you will get resistant spidermites and then you have a bigger battle on your hands.Also do not allow pets that go outside alot to be around indoor plants,mites have been known to be carried in from pets to plants. PLEASE NOTE, below are ways to get rid of spider mites, but no 2 mite infestations are identical, you almost always have to use different products to get rid of them, also make a habit to check for at least a whole month after thinking you got rid of them, that way your chances of the spider mites that became resistant breeding and then having a bigger problem. Make sure you spray your plant down very good and enough and on time.

Fungus Gnats

Fungus gnats like to feed on roots of the plants and organic matter. Adults and larvae live in moist, shady areas. The adults lay there eggs on top of the soil, near the base of the stem and takes about 4 days to hatch. The larvae will start by eating the root hairs of the plant then working their way up the plant, Fungus gnats like to eat organic matter so they will be stealing away nutrients from your plants, so its best to get rid of them completely.

Control
To control to prevent fungus from forming there are a few things you should do.

DO NOT foliar feed at night, tends to make humidity higher rather than when you water in the day the water has time to evaporate where at night will linger in the air.

Same goes for watering plants at night, wait till the morning or afternoon to water!

Keep a happy plant and will not become prone to infections. Checking plants often can aid in getting rid of any fungus that may attack other leaves and or bud!

Have a lot of air going around the plants for bad ventilation= sick plants and a breeding ground for spores!

Keep leaves away from soil making sure they do NOT touch the soil.

Keep cooler temps at night while plants are on there down time.

White flys:
They look just like tiny months, and lay eggs on leaves and or top of the soil. They are a pest in big numbers and are not very hard to get rid of, they can cause damage by sucking sap from the plants. Here are some ways NOT to get them: Do not overwater your potted plants. Allow the soil to dry between waterings and provide good drainage. Another one is to put sticky traps up when you think you may have them, that way when they get stuck you can identify them.
if you already have white flys, you can use insecticidal soap, this will help stop them from flying so it wont infect other plants.