Fan leaves falling at bottom

I have a 2x4x8 Grow tent w/2 plants under 1 400w hps. I have a 4in vent fan that Only turns on with the lights. Could that be the reason the bottom fan leaves are falling? (Because of the 12/12 vent fan cycle) or maybe it's a nute problem? Plants are 3weeks into flowering and lookin great except the the bottom leaves are turning yellow and falling off. Should I just keep the fan on 24/7 or is 12/12 just fine?

There dying because the plant doesn't need them. Leaves suck energy from the light, be thankful. With these leaves dying, the other leaves will get more energy. Resulting in more bud. I would feed them more too. Fan time is good

Did you switch straight to a flowering nutrient right when you flipped the lights to 12/12? Most flowering nutrients have VERY low nitrogen levels. That first couple weeks of bloom (the stretch phase) plants use a LOT of nitrogen. If you don't give it to them, they will take it from the lower leaves.

lower leaves yellowing and falling of is an indication of N deficiancy.. maybe add slightly more N and MG into your feed, but be very gentle with i since youre so far into flower.

So the basically, it's a normal thing and the vent fan on 12/12 to coincide w/lights is not a problem?

correct, i freaked my first time this happend as well. though i was killing my shit. but if u look into most peoples flower room. u see alot of yellowing leaves. its due to flower food having very little N in them. N is what plants use to turn green. without it they turn yellow. and the leaves fall off. just dont panic and im sure u will be happy in the end.
Happy smoking
Bubba

Check your humidity with lights off without the fan and then try it with the fan on and lights off. Humidity would be the main reason to keep the fan on all the time.

I don't know but it seems to me the vent fan should be on 24/7, there should be constand air circulation, as said yellowing leaves is usually a sign of N deficiency, tho fan leaves tend to die off towards the end of flower cycle.

I would also recommend running that fan full time, like others have mentioned. It's not the cause of your leaf yellowing, but still a good idea. Stagnant air is not welcome in my grow space, EVER. It's an invitation for molds and mildews.