[Warnberg]

There should be a way for you to flush the RO membrane, also check the filters. Sometimes if the water is too cold or if you do not have enough presure your output will be low.

What you are looking at doing here is changing out your water over the next couple of weeks. you say your RO/DI unit is producing about 15 to 20 gallons a day.

Day 1 = 20 gallon RO/DI water, mix salt with powerhead, bring to 1.025 salinity with refractometer and add heater to bring temp to within a degree or two of tank water. After mixing leave for about 4 hours and then do your change.
Day 2 = check tank parms and take notes, do not add anything, start making another 20 gallons of RO/DI water
Day 3 = repeat day 1
Day 4 = repeat day 2
Continue this for 2 weeks or total water changed 120 gallons.....