reefer dane, I see no problem with what you're doing, however be aware that slightly low pressure will only affect the production, (how fast you make water) and the rejection (how much TDS is removed), it won't "burn up your membrane". You will want to purchase a TDS meter and properly test the RO water before it goes to the DI, use the calculator here to ensure you are at least 96%. If you're not you might want to add a booster pump. This is important to know because the better your rejection, the less your DI will have to work, hence the more water you'll be able to process before they become exhausted. Note: Unless you have a color-indicating cartridge, you'll need a TDS meter anyway in order to determine when the DI needs to be replaced. Typically for saltwater aquariums, a reading above 1ppm would be time to change. Hope that helps. Shane SpectraPure Inc.