You can defeat/prevent root rot with one single nutrient, oxygen. Here's how:
How to identify root rot
Root rot can be easily identified by the dark looking and slimy feeling root system. Because most of us see the plant are above the root zone, you'll notice an overall unhealthy look about your plant, first. The leaves will start wilting (soggy looking), and start developing dry, brown blotchy spots. The overall look of your plant is looking unhealthy, it's a good idea to look at your roots. If your roots look brown/slimy, then you have root rot. Left untreated, your plant will eventually die. Although it can happen in any medium or soil, it's quite common in hydroponic systems.
It's very common for new growers to experience root rot, until they learn how to properly configure their root zone, so don't feel bad. If you catch it in the early stages, it's an easy warning from your plant that it isn't getting all it's needs met. Luckily, it's easy to fix.
What is root rot?
Pythium, AKA root rot, is a living organism (generally considered a parasite) that will thrive in a moist, low oxygen area, where there's plenty of food. Pythium will feed off the micro/macro nutrients that your plant uses, and the necrotic material of dead roots. An improperly configured plant root zone is ideal for them, because it provides everything Pythium needs to thrive.
Since we know what Pythium need to thrive, it's as simple as neglecting it, to prevent it. Since Pythium and our plants both need moisture and food to survive, we can't really deny wither of these requirements or we'll kill both the Pythium and our plants. The key nutrient here is oxygen. Out plants can't get enough, and it's toxic to Pythium. All you need to do is provide ample amounts of oxygen to the root zone, and root rot will be a thing of the past!
It's simple in theory, and practice.
How to maximize the oxygen in your root zone:
This sounds scientific, and it is. Luckily, we can control it in a non scientific manner.
Water (H2O) is made of two hydrogen, and one oxygen molecule. In between these particles are all kinds of other things that get trapped (mixed) into the water. Nutrients and gasses for instance. Like everything in the universe, water is full of energy and vibrate at a higher rate when warm, and a slower rate when cold.
Why is this important? The warmer the water, the faster the molecules vibrate, and the trapped gas particles are freed from the water. The colder the water, the slower the molecules move, and the more gas is trapped in the water. This is easily demonstrated by opening a cold soda and a warm soda. Which one will "go flat" first? The warm one will go flat first because it quickly releases the CO2 from the solution. The colder soda will take longer to go flat because the CO2 gas is "trapped" in the solution of water. It hasn't worked its way out into the atmosphere yet.
Based on this theory, the colder the water, the more oxygen it can hold. The warmer the water, the less oxygen it can hold. Note: The more dissolved oxygen your water can hold, the higher the pH!
The first two tips apply mainly to Deep Water Culture systems (DWC)
Pro Tip #1: Water temperature is key! 65-70 degrees F is the best range for our plants. 68 is ideal.
In a DWC hydroponic system, where roots are submerged in the water 24/7, maintaining proper temperature is necessary. If you grow in soil, Hempy buckets, or flood/drain systems, nutrient temperature isn't as critical, because the roots will get oxygen from the atmosphere when the root zone dries out. If you can't control your DWC nutrient temps., DWC isn't for you. Flood and drain systems will work even when nuts get warm!
Pro Tip #2: Agitate your water, and break up the surface tension so it can absorb more dissolved oxygen. Air pumps and airstones are a great way to do this. You want at least 1 watt of air pump power per gallon of nutrient solution. 2 watts per gallon is better! Adding more airstones isn't helpful, adding more watts (bigger pump) is the key. Waterfalls are a great way to mix oxygen into the water. If you can design a system that allows water to fall into the reservoir, it will be beneficial. Blowing a fan over the surface of a reservoir will cause ripples and increase surface area to increase oxygen absorption.
Pro Tip #3: Let your medium dry out between feedings. I can't state this enough. Regardless of your medium, let it dry out before watering/feeding again. This will expose the roots to oxygen it needs. Ideally, you should only water your medium enough that it will be dry within 24 hours. Coco coir may stay moist for longer, which is fine, but let it dry out between feedings. If you're using a flood/drain system with hydroton, you'll need to water every couple hours or so, but LET YOUR MEDIUM DRY OUT between feedings.
Pro Tip #4: Only water with the lights on. Because photosynthesis only happens during the day, the roots aren't using up the water in the root zone like it does at night. This will cause any moisture to remain, and become stagnant.
Pro Tip 5: Keep your roots dark. Algae will grow in the same conditions as Pythium, however some types will thrive in the oxygenated water, too. Algae could coat your roots and prevent them from absorbing nutrients, much like root rot. Because of this, be sure to keep your root zone dark. Even small light leaks could provide enough light for photosynthesis to occur in algae. So keep the light out, too. People will argue that roots can grow in light, which is true, but algae does too.
If you already have Pythium, and need to fix it:
If you already have root rot, you simply follow all the above tips. In addition, you can remove the slimy roots, let your roots air out for 15-20 minutes, drain and clean your reservoir, fill with fresh nutrients. Repeat this every day, dor a couple days until new root growth has begun, and the old slimy stuff is gone! If you had brown leaves, they won't get better, but your new growth should look healthy.
That's all. No fancy products are required. No hydrogen peroxide, or bottles of magic solutions. The nutrient manufacturers would love to sell you hydrogen peroxide (H2O2) to supplement 24/7, but all you need is an air pump, or to water less frequently.
If you have any questions, I'd be glad to answer.