3 stage RO worth the 20 bucks?

The purpose of the sediment filter is to protect the carbon block so its billions of tiny microscopic pores do can adsorb chlorine and do not become plugged or fouled. This is extremely critical since chlorine can melt the thin film composite (TFC) membrane in a heartbeat.

What you may not realize is the unaided human eye can see something that is 40 microns in size so 5 microns is really pretty big in the grand scheme of things and will plug a carbon block with sediment, silt, particulates oe colloidal materials in a hurry.

The cleaner you can keep the carbon block the longer it will last and the better it will work meaning the longer the RO membrane will last and the longer the DI resin will last. look at it as a complete system, each filter depends on the quality of the one before it and the result is a much lower cost of ownership over time and much better water quality. I use and recommend a 0.5 or even 0.2 micron sediment filter which is 10x or more better than a 5 micron filter and provides the best protection for the carbon block and membrane.

Water is the single largest ingredient in a reef system and everything depends on it so don't skimp on a RO/DI system and water quality. Look for a system that goes a step above, not one that is just mediocre in quality and nothing special. Your tank will thank you!