Can't find any spec's on that membrane. Wonder what the testing pressure spec is, what what pressure you are providing it.It's a
SpiroPure SP-RO-400R RO Membrane Filter Replacement 400 GPD
SpiroPure SP-RO-400R RO Membrane Filter 400 GPD - $20.39
The SpiroPure SP-RO-400R Reverse Osmosis Membrane Filter Replacement 400 GPD for undersink RO systems for a high reduction of contaminants from your water.www.allfilters.com
Because of your high (and untreated, correct?) hardness, you'll want to run that membrane at something like a 3:1 or higher ratio of concentrate to permeate. So let's do some math here.
Permeate production: 400 gpd (you're probably not getting this much but let's use 400 gpd for sake of discussion)
Concentrate production at a 3:1: 1200 gpd
Feedwater flow = 400 + 1200 = 1600 gpd.
1600/1440 = 1.1 gpm
And you're trying to do this with 10" x 2.5" prefilters with 1/4" housing ports. That is not a good fit.