View Full Version : RODI unit low vs high GPD?

Lord,Darth Bane
09/02/2017, 01:14 AM
So I am looking for a RODI unit for my 100G SPS system. The TDS of my tap water is 117ppm.

The place I intend to put the unit has no socket nearby so I want an unit that doesn't need electricity (no booster pump).

So, questions:

1. Is GDP high better than low?

I have checked out many units with GPD from 50G to above 400G. Most branded one such as Kent, BRS, Spectrapure and Vertex are from 50G to 200G. But those cheaper "ebay type" have 400G or even 800G.

They all "claimed" no booster pump is needed. But do the higher GPD one needs higher pressure to work? In theory a higher GPD membrane has bigger surface area which allows more "pure water" to pass through.

I am confused and I have asked several sellers:

A only has 125G. He said those 400G without booster pump are "low pressure membrane".

B sells units from 50G to 400G. He suggested only to use 50G without booster pump. (The higher one like 100G may not work without booster pump).

C sells both 125G and 400G. He said both don't need booster pump and the 400G could produce more water than the 125G.

D sells 100G to 400G. He said 400G needs much higher pressure to work.

.......I am confused. So what is true?

Is it higher GPD needs higher pressure to work?

And for those high GPD membranes that work with low pressure (low pressure membrane), their rejection rate is low? (which means the output TDS is higher?)

I am just curious, why some cheaper units have much higher GPD like 400G but our branded aquarium used ones are usually only around 50G~100G?

So for a unit without booster pump, what GPD shoud I choose?


Thank you.

09/02/2017, 02:53 AM
does your home have low water pressure?
If yes you might need booster pump but in most cases it's still not needed
If no then great you don't need worry yourself about booster pumps

No hobbyists needs a 400 gallon per day unit that's way to big

But to help you make a decision get the RO WITH DI
Plus 75gpd unit ( is all you need ) from Bulk reef supply its the most used unit in the hobby
Comes with pressure gauge tds meter and back flush

Lord,Darth Bane
09/02/2017, 04:11 AM
Thanks. I haven't checked but I think the pressure is enough as the tap isn't slow.

Another question:

Should I get an unit with 3 or 4 canisters? I read that there are different "recipes".

5um or 1um sediment filter > carbon block > RO > DI


5um > 1um > carbon > RO > DI


1um > carbon > carbon > RO > DI


1um > carbon > RO > DI > DI........

Would an additional canister reduce the water pressure going into the RO?

09/02/2017, 04:49 AM
Buy the BRS 75GPD RODI unit but you will want to find out if your water company uses chlorine or chloramine and make sure you get the proper unit. One has filter for chlorine and that filter will not filter out chloramie. If you local water supply using chloramine then you will want the unit with the chloramine filter. Either way the 75 GPD BRS RODI unit is perfect for what you need. Buy it and don't buy the cheap eBay ones. Most of those usually have inferior filters.