Is the yellowing on new growth or old? if its on old dont worry about it to much unless it looks like its spreading going up the plant just keep an eye on it. If new growth is turning yellow that is very bad could be PH levels or some sort of deficiency. Since your using 5 gal buckets for clones you probly dont want to water them untill water comes outa the bottom so testing ur PH through your run off water with PH testing strips is going to be a bad idea. So go buy urself a PH pen.Your PH should be in the neighborhood of 6.0 to 7.0. If its way higher or way lower then that then thats whats most likely whats causing ur yellowing. Its also a good idea to test what ever water source your using cause eventually your soil will become the same PH as its water source. correct me if im wrong :P