The fastest way to kill growing seeds is to let them dry out. In the early stages of germination, you need to make sure that you keep your soil moist at all times.
You will probably have to thin your seedlings after they get their first set of full leaves, to the space that is recommended on your seed packets.
Check water and feeding requirements for your particular plants.
If you are growing basil, once it is about 4-6 inches tall, let the soil dry out a bit between waterings. Also, if you are growing herbs don’t feed them too heavily. You will get the most flavorful herbs if the soil is somewhat poor in nutrients. The same is true for nasturtiums, which will also flower more if fed less.
Enjoy your plant pool party!