4 young tomato plants die
 

recently i had 4 young tomato plants die using a 16 bottle system. plants wither & die. I notice the stem or stalk tapers to a smaller size at the point it enters the perlite. I had changed watering times from every hour to every 30 min. could this be the problem? all other plants in this system are doing fine. plants lost were about 4 inches at time problem occurred.

That is what is known as Damping-off - a fungus disease that attacks young seedlings and cuttings causing them to rot at the base: Over-watering is the main cause of damping-off.