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