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For all you salties, and maybe a freshie or two, how often would I have to replace the membrane cartridge in a 3-stage RO unit that filters 24 gpd? Now, I would only be doing about 10-15 gallons a week (for a partial PWC on my 55....I would still be using mostly tap), and my water has a rough gH of around 300 ppm or 16 point something degrees. I know, very high, but it is what it is. I've seen the initial cost of the unit, it's the cost of the replacment membrane that nearly made my eyeballs fall outta my head.....it wasn't too much cheaper than buying a new unit (well, I exaggerate a little, but not much )

So, how often would that thing need to be replaced?

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~~Melissa~~
"Beauty is in the eye of the beer holder."

Heavily-used RO purifiers should get their membranes changed every year, but you can get a lot more use out of yours. I don't think you'd have any problems waiting two years before changing out the membrane. Coralife recommends replacing their own membranes every two to three years, so even if you're getting a different brand that's probably in the ballpark. You might be able to go longer than two, but at that point there's a risk that your unit will become much less efficient.

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And when he gets to Heaven, to Saint Peter he will tell: "One more Marine reporting, Sir! I've served my time in Hell."

Best way. Get a TDS meter and test it. Then you know for certain when it needs changed. TDS meters are cheap. I got mine for free with the unit.

Okay, yeah, the one I'm looking at is the Pure Flo II by Coralife, so at least maybe two years...okay that will work. Not so bad in that context. The would be like 60 some bucks every two years instead of every six months or so....no problem.
Thanks sirbrooks!

Sham, on the TDS meter is it the finished product that your testing and when the TDS count starts to go up too much you know it's time to change the membrane?

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~~Melissa~~
"Beauty is in the eye of the beer holder."

Yep you run some of the RO water into a cup and then stick in the tds meter. It will give you the total amount of minerals and other substances that are making it through your ro system. If it goes up then something is wrong, a cartridge needs changed, or your main membrane needs changed. Very handy to have. The exact value that is acceptable before you need to change things depends a bit on what you want the water for. If your just softening your current water and not really looking to remove anything bad then a higher value even near 100 would be acceptable. If your looking for as pure of water as possible such as for a marine tank you probably want it below 5-10. The exact RO system, your desired use for the water, the hardness of your tap water, and how often you end up using it is all going to have an effect on how often you decide to change the membrane so it's not very easy to give an exact number of months or years.

Okay, all handy info for sure. My main concern is that I would have had to change that membrane out in months....if it can last at least a year, fantastic. Two years....manna from heaven. All I'm looking to do is soften my tank water a bit. With a gH of 300 ppm it gets to be a little much seeing as how I don't have Africans. It coats my tank in crud and I don't think peat in my filter is going to make much headway. I'm only going to do about 10 gallons a week. Just soften up my water a bit, I still plan on using mostly the liquid rockfrom the tap...that way, I can still have some kH and the minerals that the fish need. This is a FW set-up with angels, rams, and platies right now....I'm adding tetras and a pleco later. 'Preciate the help!

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~~Melissa~~
"Beauty is in the eye of the beer holder."

You won't need much of that liquid rock. It took me 2 gallons of really good RO per 1g of water with 18degree hardness to actually soften the water and lower ph values by a noticeable amount. If I add 1 cup of tapwater to a 10g tank of ro water it buffers the ph beyond what I want for my rams. I went to using commercial buffers and minerals to add hardness to the ro water instead. It makes the results much more predictable and I can fine tune it better instead of ending up with either ph 6 or ph 8. Plus I don't have to worry about any other junk that is in my water.

Yikes! Well, I guess a 24 gpd RO will suffice since I don't change that much during my weekly water change. Usually, when I do a control test in a cup, if I do 1/2 tap water and 1/2 RO water (from my cooler) the values land pretty much where I want them. So, if I do a 30% water change on my 55 which is 16.5 gallons....I figure on doing about 8.25 gallons of RO and 8.25 gallons of tap water. I guess I can adjust it later if I don't find it adequate, but max amount I'm looking at is 17 gallons of RO per week.....if I do start doing my water changes with 100% RO. I guess, if I did eventually start doing PWC's with 100% RO, then I would have to add buffers and minerals because it would eventually be all RO in there.

In any case, I'm thinking an RO unit of this size should suffice and perhaps the cartridge will last at least a year.

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~~Melissa~~
"Beauty is in the eye of the beer holder."