Ok, I have seen lots of folks online and on this forum in general make comments and such that would lead me to believe that they are more trusting of the manufacturer to sell you something that runs perfect out of the box...Truth is, there are many things you need to take into consideration when you are purchasing a RO/DI unit or upgrading your current one.

First thing that I will talk about is the membrane itself since it is the heart of your RO system. Not all Membranes are made the same. Membranes in general cost about the same to manufacture whether it be a 25gpd membrane or a 75gpd membrane. Dow Filmtec membranes are the industry standard in membrane quality. I recommend them for any system no matter what the make or model as long as you have a standard membrane housing they will fit. Be aware that you can purchase a 25-75gpd Dow Filmtec membrane for around $25-35 depending on the store anyplace online. The ones that the online fish stores sell for $50-100 are not Dow Filmtec Membranes and are not as good of quality. They are all gimmicking membranes especially the ones that say HI-S or Hi Silicate removal. They charge you a premium price for a Chinese made membrane. Filmtec are Made in USA.  Do not exceed a Dow Membrane or any other membrane that is over 75gph. The standard rejection rate on a 75gph membrane is 95-98% meaning it gets rid of that much TDS out of your water. Anything over 75gph offers to0 little of a rejection rate and will eat through DI resin like crazy. 100 GPD membranes have an 88-90% rejection rate and are typically not recommended for even drinking water systems but more for pool and spa use. Membranes be it a 25gpd or a 75gpd are the same size and fit in the same housing. With that being said you are also paying a premium for systems that offer a selected GPD output. Typically a RO system that runs 25gpd is a lot cheaper than a 75gpd system even though there is not a single difference in the system design or other components other than the membrane capacity. With that said you can take a 75gpd membrane and put it in your 25gpd system.  You would just need to change the flow restrictor which is a simple $4 part. When you see systems that claim to run 80-90 GPD this is done by membrane selection. Even though a membrane is rated for 75GPD at 60PSI it may run more than that at 60PSI which is what these companies put in their systems, hand selected membranes. Some companies have a process to boost a membrane up to 90gal or so a day such as Spectrapure, they buy Dow Membranes by the lot, run them through a proprietary process and hand select the ones that meet the output standards, slap a Spectra Pure label on them and sell them as 90gpd membranes. Even these high output membranes do not sell for over $35 and are considered the very best you can get.

I should also mention that pressure has a lot to do with the gpd output of a membrane. For a 75gpd membrane they rate the gpd at 60psi . If you lets say bump that pressure up to 90psi your going to get around 100gpd out of that 75gpd membrane. This is what I do with a booster pump. However typical household pressures are usually 60-70psi so getting the 75gpd or better without having to boost the pressure is normal.

Flow restrictors do need to be changed if you decide to run a higher capacity membrane than what came with your system. The flow restrictor is located on or in the waste water tube coming off your membrane housing. You will need to get a matching flow restrictor to your membrane output to ensure you are getting the right waste to product ratio. A capillary tube style is the best as you can trim it to get the perfect ratio. Even if you have or get a unit that you don’t plan on changing the membrane capacity on, still check that you are getting the correct waste to product ratio as the stock flow restrictor may be set for a idea incoming pressure of let’s say 60psi….If you have higher or lower pressure than 60psi it is going to affect your ratio using the stock flow restrictor.

Waste to product ratio with no exceptions is going to be needing to be at 4:1 , that’s 4 parts waste water to 1 part product water. You can be green as you want to be, but if you’re running a RO unit this is what you’re going to have to live with. That means for every 1 gallon of good water that goes into your collection tub, you are producing 4 gallons of waste water. This needs to be done because it produces the correct back pressure on the membrane itself and flushes the membrane properly, making the membrane be as efficient as it can be and making the membrane last many years. A very short hair under 4:1 or over 4:1 is ok but as close to as u can get it is great. A flow restrictor controls your ratio.
 It is best to have 2 on hand for each season as water temp effects your ratio and it is recommended to change the flow restrictor for the summer and winter months.

Check valves are small one way valves that allow water to flow only one way. An RODI unit should have a check valve fitting on both the product and waste output of the membrane. This is key as it does not allow water to come back into the membrane from the product line or allow waste water to creep back into the membrane from the waist line. This alone can ad many years to your membrane. They do make inline check valves that can go directly on the tubing or they sell the quick connect fittings that have the check valve built in that would replace the fittings on your membrane housing. They also offer a solid foot for shutoff valves and such to build pressure on to work right if you decide to add that to your system.
They do make these in the 90 degree elbows also just look around for them if you need the elbow as most systems have that on them.

Well I know I have probably missed some stuff or forgot to ad sometime so if you have any questions feel free to ask.

Hope this has helped !

Thank you, I just like to inform the best I can and help people stop waisting money if they dont have to.

If you have half a brain you can build your own top of the line 75gpd unit for right around $120 that will be better than most the stuff they sell. RODI units are not rocket science. If you can turn a screwdriver and use your fingers it is about a 30 min job to build. I can give you a detailed parts list so all you have to do is order, you wanna call me up and I'll talk you through it im here to help. If you go the DIY route, it will have the right membrane, the right sediment/carbon block/DI right off the bat.

But no, RO only is better than nothing but there are still trace amounts of stuff in the water and depending on how bad your source water is, there can be a lot.

It does not cost any more for a 75 than it would for a 50gpd a 50gpd membrane is sometimes more expensive as they dont produce as many.