There are quite a lot of hydroponic applications, and each of them requires different specifications. For this reason, we cannot give you a general number for the GPH, as it varies depending on the situation. We suggest that you first calculate the amount of water you want to move and your system’s head height. Then, you will get a pump double that size.  Even if you have a much bigger pump, you can still quickly fix it by adjusting the water flow. On the other hand, if the pump is smaller than what you need, there will never be enough water.