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I'm going to go with tank 3 since it's the lowest and a tanks are connected. Sorry, not going to resteem, though.

no problem ..thanks for replying

I am going to say 2 but it depends on the speed of the water being piped into the system, the connection between 2 and 4 is smaller than between 5 and 2. Tank 2 will be pretty full time water flows out to tank 4 so there is a chance it fills before tank 3 gets a chance to.