First, always make sure you get coco/soil/rock/cubes or any medium from a reputable source. No exceptions.
The first thing you want to do, is buy sticky yellow gnat cards inside of a holder that can be placed right above the soil. The ADULT flying aphids will get stuck, ending their shit of a life... (cocksuckers). You will be astonished at the THOUSANDS of them that you had no clue existed. This will limit the number of eggs being layed.
You want to make sure you are changing the cards once they fill up, some people don't and it slows the process.
Misinformation is brutal. You CAN'T get rid of aphids that have already infested a rootball, It's impossible. You can only hinder the scale of their reproduction.
DE is great for helping slow the flyers from being able to get out one they evolve, however they can still get in through drainage. You must also apply DE everytime you water, as it does wash away. (Not practical for coco)
You can't kill them, you can only prevent them and control them.