[Support request] Question regarding a growth issue

Hello I am experiencing a problem with at least one of my plants,it may be starting on a second one. Currently I have three plants growing. Two have been experiencing a very steady growth while third,being a different type, seemed to struggle to keep up. Although all plants have been healthy after my initial problems getting the system started everything had been going very well. When I started this grow I had to use what I could get so the medium used is a combination of quality soil and washed gravel with about 4 or so inches of washed gravel at the bottom. About two weeks ago I noticed that there was yellowing of leaves occurring on one of the three. This plant however appears to be the middle if you would in terms of size out of the three plants. As I mentioned the two larger plants are of the same species and were relatively close in every way but the plant experiencing the yellowing is slightly smaller than its sibling. I thought at first that this was a sign that it required an increase in nitrogen. So I added an extra 25% in nitrogen and waited. After a week and a half of watching and waiting the sibling plant has started showing yellowing as well. Yesterday I pruned all yellowing and anything else that should go off the plants. I’ve checked the reservoir and it is sitting at 6.2 and 1450ppm. I’m no longer sure that this is an issue caused by the lack of nitrogen but I am concerned that they are already suffering from being root bound and it’s only June. I need to make it to harvest in late September to late October. Help please