4x600W in 6 inch cooltubes, do I need 2 fans?

So in the garden there is 8 inch rvk extracting air thru the filter and 6 inch rvk pushing air thru cooltubes in separate circuit to reclaim some heat. One 6 inch fan is not enough I think, draw at the and of duct is weak, not like for 6 inch fan. about 10 meters of ducting was used for this circuit, should I just add another 6 inch fan?

I would increase fan size to 8". You are fighting the pressure along the walls of the ducting. As the duct length is extended the pressure from the walls is increased, slowing the flow. Read about Venturi Effect.

Thanks, I solved this one, just changed air filter on the inlet, it is blowing nice now

It's not the venturi effect, it's flow loss from friction..well, flow reduction due to turbulent flow along the edges of the ducting.

Venturi effect is caused from reducing the size of piping.