It is essential that the seedlings have sufficient space to branch out, which is why thinning is an option to consider at some point. Tomatoes require plenty of light. That could include sunlight, but you can also use an artificial source to ensure at least 15 hours of light daily. Watering depends on location and tomato type. Seedlings need a tiny portion of water since airflow is critical for their development. You can consider watering tomatoes every day once they develop. Try to decide based on the temperature. The experts estimate that tomatoes grown in the garden need approximately two inches of water weekly.