Seedlings are delicate little things and need a tender touch. This includes their fertilizer.
I start my plants off with a weak solution of 2-5-2 Water Soluble Seaweed Powder from T & T Seeds. T & T is kind enough to send a free package of their Dry Seaweed Concentrated Fertilizer when you buy seeds from them. I use it for my seedlings and they appear to love it – they only need a shot of fertilizer once a week. If you don’t have Seaweed Fertilizer you can use any weak solution of fertilizer to give your plants that boost they need, starting after they have formed their first “true” leaves.