You will want to get one that is large enough for your needs. I have a 125g tank, so I went with a 100gpd (gallons per day) RO/DI unit that I feed into a 50g tub with heater and powerhead. This way I always have enough water waiting in case I need to do a major water change. It just makes life so much easier. As far as enough pressure, most houses are plumbed with the proper water pressure, usually around 45 pounds. I have it on my cold water side of the washing machine. I can open the valve and it goes right into the unit, the spent water goes into the drain tub for the washing machine. Works great. I periodically test the water with a TDS meter to make sure the water is pure. Good luck.