I have a planted 2' tank which I dose daily with TPN+ and easy carbo. Would it be feasible when doing water changes to use this water in another tank which isn't dosed (4ft). The other tank just get nutrafin plant gro every now and again.
It's not a good idea generally to put the dirty water from one tank in another. That's because you will be transferring the organic waste from the first tank to the second. It's the buildup of organic waste that we are trying to get rid of in the first place, that is the reason for the water change, so you will be putting more waste into the second tank. This doesn't even address the possibility of passing on pathogens to the second tank. You might get away with it for a while but this is really bad juju all the way around and the Tank Gods will surely not smile on you. I've got a better idea. Why not just dose the second tank the same way you dose the first?