The pump size for your waterfall depends on the size of our waterfalls. For example, if you have an 18-inch wide waterfall, you need to install an 1800 gallon per hour pump. However, if you have a gentler waterfall then you can go for the pumps that spew 900-1800 water per hour. But, if need a more aggressive flow use a 2400-3600 gph garden pump. In short, the size varies depending on how big your waterfall is.