I've got the smaller NFT tank from growell (Gti205 20L rather than 40L). I sucessfully grew 4 plants under a 400w HPS no problem - those NFT tanks are great, growth rate in incredible.

With the grotank you have you'll easily be able to fit 4 plants in as it's twice the size of mine, maybe even 6-8 in my honest opinion.

But yes, exhaust fan is a must
Bluefunk Reviewed by Bluefunk on . NFT Setup - HELP ME!!! Hi, Just a few questions, i am a complete novice. Gonna start a grow using NFT. 1. Am gonna buy budget kit from Growell.co.uk, includes 250 w hobby bulb, is this bulb good enough for 2-4 plants?JK What sort of yield would each plant produce? 2. Gonna grow in a Growell Grow Tent 1m x 1m x 2m, Do you recommend any ventilation products to help with heat distribution? Rating: 5