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Source Water: Choosing and Maintaining your RO/DI Filter

Tenji

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Tenji is pleased to present a series of exclusive articles for Reef2Reef members. We will be delving into the various aspects of reef keeping, focusing on tried and true methods that can be implemented by aquarists of all levels.
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Our first installments will be concentrating on the most basic aspect that is frequently overlooked; your source water.

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The most important start to a reef system is quality source water. As most of you know this begins with using a reverse osmosis deionizing system (RO/DI). It is important not to get caught up in the number of stages, or any aesthetic nonsense. You want something simple and affordable to maintain while providing water with 0 ppm total dissolved solids (TDS).

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Remember that TDS measures every solid dissolved in your water, so while a level of 1 ppm TDS might not seem bad, we don’t know what that 1 ppm of dissolved solids is without additional testing. It could be copper which is toxic to invertebrates in high enough levels. It could be the source of your never-ending algae problem.

Basic filters for an RO/DI system consist of (in order): sediment, carbon block, membrane, and deionizing resin (DI). Note that every filter is used to protect the next filter in line. You could run your tap water directly through DI resin and produce 0 ppm TDS water, although you would likely be replacing the DI very frequently which can get expensive quickly. Most of us will have source water with chlorine added, which a good carbon block will remove. However, many municipalities are using chloramines nowadays, which will sneak through and damage a standard carbon block. Specialty carbon blocks for chloramine removal should be used in this case.​

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As previously mentioned, we measure the quality of our freshwater based on total dissolved solids (TDS). Inline TDS meters are commonly sold for RO/DI units, but it’s important to have a secondary tool to ensure our TDS readings are accurate. Entry level inline meters can skew over time providing false measurements, which can send us down a wild goose chase. Adding a handheld meter to your arsenal is a great idea for long-term reef keepers. Handheld meters are affordable, and since they are not constantly submerged in water tend to remain accurate for longer periods of time. Most handheld meters measure temperature as well, which you can use to double check your reef or quarantine systems.


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Maintaining your RO/DI system is just as crucial as finding the best filters suited for your application. A properly fit RO/DI system should not require much maintenance, but be sure to keep up on your filter replacements. As soon as TDS creeps above 0 ppm, change out your DI resin. If your unit is fitted with a pressure gauge prior to your membrane, a drop in pressure is indicative of your prefilters (sediment and carbon) requiring replacement. If you do not have a gauge, it's best practice to prophylactically change your prefilters every six months. Once your DI replacements become more frequent it's time to look at your membranes efficiency. Test your the water's TDS pre and post-membrane. Compare the results to the manufacturer's claimed rejection rate and if drastically askew, it's time to replace. Generally, membranes can just be prophylactically replaced yearly. Be sure to flush new filters with downstream filters removed, or bypass them entirely. The picture below shows what comes out of a carbon-block after flushing; this could be prematurely clogging your membrane!

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Everyone’s source water is different. The water coming out of your tap might even be different than what comes out of your neighbors. This is especially true in homes with older plumbing. Many off-the-shelf units will work fine for hobbyists; however, you might be replacing filters significantly more frequently than you have to. Try working with a company dedicated to selling RO/DI systems since they’re more likely to sell units best suited to their client’s source water. Make sure your supplier is asking you about your source water and pulling your township’s water report from the internet.

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Finally, we all need somewhere to store our RO/DI water. Keep a tight-fitting lid on your water, as it’s so pure it will literally grab compounds out of the air which can raise your TDS prior to even being used. Brute trash cans are an easy and affordable vessel for your precious water, but of course, you can source other shapes and sizes from various suppliers. Any reservoir intended for potable water use is fair game.

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In our next article, we’ll take a look at the second most important thing for your reef’s source water, your salt!
 
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revhtree

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PopaSmurph

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I've never heard of tds creep coming from air? My LFS even told me to put an airline into my resivor to keep oxygen in my water. Now I need to go buy a handheld tds meter... thanks lol
 

ReefInskeep

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So what companies sell custom RO/DI units based on source water, city reports, etc. as you mentioned? And, is that more expensive than basic units from BRS or the like?
 
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Tenji

Tenji

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I sure would like an explanation of the plumbing involved, too.
Most units would be plumbed similar to this. Note the colored tubing called out is specific to the source, @Buckeye Hydro.
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I've never heard of tds creep coming from air? My LFS even told me to put an airline into my resivor to keep oxygen in my water. Now I need to go buy a handheld tds meter... thanks lol
It can be brutal in an old dusty basement! There's no need to oxygenate RO/DI water (unless you're preparing a freshwater dip for parasite control), but it's generally a good idea to keep mixed saltwater oxygenated.
So what companies sell custom RO/DI units based on source water, city reports, etc. as you mentioned? And, is that more expensive than basic units from BRS or the like?
The basic units themselves are all very similar, it's the filters inside that can be fine-tuned. We frequently use Buckeye Hydro, who' a sponsor here on Reef2Reef. BRS should certainly be able to help choose the correct filters. Unfortunately, we find many people don't ask any questions and simply purchase the most common unit paying no attention to tap water parameters. Of course, this will generally still produce 0 ppm TDS water, but may be burning through filters prematurely.
 
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Waalker

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This is great, I think more awareness should be brought to this topic. I have been guilty myself of not having a TDS device and then combine that with not having great habits replacing the media in the RO Unit (Mostly due to ignorance) and BAM! One day you get it together and find that you have been putting water with 9 TDS in your system! =( Everyone can see their DI resin change color and that's all well and good, but I think it is much less likely known when to replace your sediment , carbon blocks, and membranes. I really like that you called about some times in this article, thanks!
 

Fishinwall

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How do you ensure your inline tds meters remain accurate? Is there a process to clean and calibrate?
 
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Tenji

Tenji

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How do you ensure your inline tds meters remain accurate? Is there a process to clean and calibrate?
Consult with your supplier or the manufacturer of your inline meter regarding calibration. There are a couple different calibration solutions used depending on the model. To clean probes soak the electrodes in vinegar or diluted muriatic acid, then rinse.
 

Elgringodiablo

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Is it worthwhile to run ICP tests on your source water and tweak your RO/DI system accordingly? I've considered switching to a 3 canister DI setup, with anion and cation run separately.
 

2CC's

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Always great to get a refresher on RODI. Question - My water comes from a private deep well - do I need to use carbon blocks prior to the membrane (seems they are used primarily to take care of the chlorine?)
 
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Tenji

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Is it worthwhile to run ICP tests on your source water and tweak your RO/DI system accordingly? I've considered switching to a 3 canister DI setup, with anion and cation run separately.
We use ICP tests for source and RO/DI water fairly frequently. Silicates, for example, do not show up on TDS meters. You could be producing 0 TDS water that still has silicates high enough to feed nuisance algae. With that said if an aquarium system is in great shape, and standard RO/DI filters aren't being exhausted faster than expected, we would not use ICP.
 
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Tenji

Tenji

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Always great to get a refresher on RODI. Question - My water comes from a private deep well - do I need to use carbon blocks prior to the membrane (seems they are used primarily to take care of the chlorine?)
We would generally still use some type of carbon for your application. While carbon is generally used to remove chlorine, it also removes volatile organic compounds (VOC's) among other contaminants that would otherwise put a higher workload on your RO membrane.
 

Bow69

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Nice article. This is definitely IMO the most overlooked aspect in Reefing. It all starts here.
 

40B Knasty

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We don't take care of fish. We take care of water. That was the first piece of advice I got from a friend that has been doing this for 15 years. It truly is the best understanding to the hobby.
I use a Zerowater filter to get .000TDS from my tap water that I filter through a 5g bucket into another 20g bin on casters to roll back and forth from the tap to my mixing station. I replace the 1 filter every 2 months give or take if there is any tank issues. Using an MJ1200 with an aeration line that is at the end of the powerhead during mixing. Dose some Prime. Water is good to go!
 

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