Advice re Hydroponics please?

I only use rainwater then add Maldon Sea Salt (apparently the cheap table salt contains caking agents and other nasties). Last year I managed to get the seed to germinate in normal seed compost but they never looked happy and just gradually faded away. So this year I've stuck the plants into builder's sand, about 50% of them have taken and are growing very slowly. Every now and then I soak them with my salt water mix. I'm not giving them much TLC, just every few days when I notice them, so maybe that's why they aren't flourishing.

Bear in mind I've had no success so I can only speak from failed experience