We aren't allowed to post Ebay links, but that doesn't matter. There is no "best" fertilizer. Aquatic plants need nitrogen, potassium, phosphorous, carbon, and trace elements. The nitrogen is best if it isn't from ammonia or urea, because it takes very little of either of those before you have problems. Good aquatic plant fertilizers use nitrates for most of the nitrogen in them. Economically, the "best" fertilizer is basic chemicals, which are very cheap, potassium nitrate, mono potassium phosphate, and a trace mix like CSM+B or Microplex.