I have 3 tomato plants of about 80 days old. All of them have small tomatoes on them :-). Unfortunately, one of them has all yellow leaves that are falling off. I have read that some of these are to be expected, however all of the leaves seems to be problematic (see picture below).
You need to add a little liquid nutrient to the water in the unit. Tomatoes use up a lot of the nitrogen and magnesium in the pods so a little help is needed after a couple months sometimes. Clip those leaves off since they are still using up the plant’s energy. The stem isn’t brown so don’t think its fusarium root rot.
Just to let you all know. We have a technique in our lab which greatly improves tomato growth. It is related to soil aeration and avoiding rootbound. Let’s hope we can have the new thing out before Christmas 2019. Sorry for the secrecy