What nutrient product should I use for my hydroponic tomatoes.?

What is your growing media? Most people water just to the top, maybe a little more. How often you water depends on your media, some holds water much differently than others. In any event, you want to keep it slightly moist, and only start to dry out at the top. The water does more than just water, it also draws fresh oxygen down around the roots, an important factor for growth. Many systems water 3 or 4 times a day – under lights, start your first watering when the light first comes on, then water at intervals in the day, with your last watering about 4 hours before the light turns off. You don’t want to water at night, this causes problems with fungus. There’s no reason to run the pump any longer than it takes to get the water to a high enough level, although most timers have a minimum “on” period of perhaps 1/2 hour.

There are probably 1000′s of nutrient mixes, and people that will argue for each one. I’d look for what you can find locally. It’s fine to add a little organic material to your hydroponics, I like to add bat guano and seaweed powder. If the plants aren’t growing fast enough, sometimes I’ve added a little extra ammonium nitrate to the mix. This is getting to fine points, and you’ll want to do some reading up on it. It’s also important to get your local water analyzed, if you’re on a well or someplace that the water supply might have a weird hardness or other mineral problem or ph imbalance, this can affect the nutrients.