Thanks, yo!
Ask them if your water hardness is too much to grow your tomatoes in (insert type/brand of soil you plan to use).

The runoff pH tells you the soil pH in this manner:
For example you know your tap water pH is 7.0. You pour some through the pot and catch the first cupful out the bottom. You test it and find that the runoff pH is OMG!!! 6.3!!! Aieeeee! Somehow in transit the pH dropped a FULL POINT!!! You then guesstimate in your most scientific fashion that your true SOIL pH is close to 5.5 (6.3 being the average of 5.5 and 7-hush up, I'm rounding-, and actually 5.5 is not an outrageous number if you are using a peat based medium). To remedy this, you adjust the rest of the flush water up to 7.5 tops- never go higher; it's stressful to da plants- and keep flushing until it comes out at 6.8. The pH you want is between 6.5 and 6.8 in soil, and if your soil tends to go LOW you adjust to the higher end of the scale.
Sorry I am in a burrito-induced afternoon stupor so that might be hard to follow.