Maybe blossom end rot, due to CA deficiency. Below are some pictures. If it looks like this, what kind of nutrient are you using, & what type of calcium are you using?

Here is a link that might be useful: Blossom end rot

Thank you Baci. That is exactly it. I use a three part nutrient formula called Flora by General Hydroponics. A web site I found said that blossom end rot is usually most severe following extremes in moisture, either an increase or decrease. Last week when I changed the nutrient solution I leached the pot maybe too well. And I increased the time of the waterings from 10 min to 15 min. I don't know if that is considered severe.

It sounds like you have identified your cause. Besides waterlogging high salinity might be another factor. You probably have the right EC, however.

Another consideration is the variety of tomato you are growing. The plum tomatoes sold in my local markets often have BER on them.

The link below has an article on BER, which you may have seen. Some key points are:
"The induction of BER occurs within 2 weeks of fruit set when the Ca concentration in the fruit is at its lowest
There is a clear genetic influence in the susceptibility of different cultivars to BER
Plum tomatoes are more susceptible to BER than round tomatoes, and no BER is ever observed in cherry tomatoes
Fruit from cultivars susceptible to BER generally have lower Ca concentrations than those from non-susceptible cultivars
A higher incidence of BER is also associated with an earlier onset of ethylene production cessation of growth and premature ripening.
Âspraying Ca directly onto young fruits is recommended for the prevention of BER."

Since you have probably identified your cause the main thing now is to correct it. For that, I will let others put in their two cents & refer you to other posts. You may need foliar sprays  possibly Ca Mg sprays. Since you are using GH, they probably have a Ca Mg product. There is something to be said about using compatible products. I do see milk recommended from time to time. If you choose to try that you might consider skim milk - the fat in the whole milk can later smell.

Here is a link that might be useful: BER article

BER in a hydro setup is either caused by a calcium deficiency or reservoir temps too high