I've run anywhere from 400 to 1400 watts in a DR120. I'm using a 6 inch centrifugal with a speed controller to adjust flow. If you go with a 6 inch size it'll be more than enough for a 400. Not knowing your ambient temp, I hesitate to recommend anything smaller. I'm in USA and not familiar with the brands you mention. Better to buy a little over=sized, it'll give you options for more wattage should you decide to go that route.
HTH
EvilCartman Reviewed by EvilCartman on . Tips on kit - DR120 I am going to buy a DR120 in the UK. What extractor fans should i be looking to get? I have a 250W HPS and high temperatures are not such a problem. i have been growing in a closet and feel it's time for an upgrade. i haven't controlled humidity before, so i am looking to get one of those temperature and humidity monitors? any way i can hook up things to run automatically by themselves? e.g. maintain temps at 75degrees F? maintain humidity at 50%? so the extractor fan kicks in only when Rating: 5