I’ve recently purchased an in line KDF filter (the one picture below) and I’m scratching my head trying to figure out how I can best plumb it. The filter is meant for RVs and boats and is meant to be used when you are filling a tank with water (as supposed to what I’m doing, filtering between my water tank and tap). The filter comes with both hose fitting ( won’t be using) for both ends as well as barb fittings (will be using) that fit into my 3/8 ID tubing I’m using for the system. I’ve looked at water filtration faucets and they seem to need to use 3/8 OD tubing, which is much smaller than what I’m using for the filter. Here’s my question, would it be possible for me to attach my filter to my hot water line (I don’t have hot water in the van) or would my best option be trying to configure a way to attach the “out end” of the filter to the small filtered water faucet? I suppose I could also try to get smaller barbs on each end of the filter but my system uses a t bard for the regular unfolitered water going into the cold water hose, the filtered and my water pump. I’m not sure if I’ll be able to find a t barb that is 3/8inch on two ands and a much smaller size on the other. Hopefully that all makes sense, let me know if you have any ideas! I can attach more pictures if needed! 
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have you checked if the filter is suitable for what you want? It's a high-flow filter designed to be in a mains water line. It might not do so well on the output side of the tank with much smaller flow.– HobbesCommented Aug 24, 2018 at 7:30
3 Answers
First, I would recommend filtering pre-tank as it's designed because that will eliminate sediment and contaminants in the tank. Long-term that's something that will create additional maintenance.
That said, what is the ID of your 3/8" OD hose? From what I found, it's usually 1/4", and you can find Ts that for that. Here's an example of one from Grainger. They're on the expensive side, but it shouldn't be too hard to find.
As a final recommendation, you can also check over at the Home Improvement Stack. They may have more helpful knowledge on the various aspects of fittings, hoses, and best practices for plumbing. But by no means does that suggest any advice you get here is bad or lacking. ;)
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It seems the question was actually migrated FROM diy.se - not sure why, it seems a better fit there than here!– Nick C ♦Commented Aug 24, 2018 at 8:59
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Interesting... I mean the topic does kind of straddle both stacks. Commented Aug 24, 2018 at 18:20
just buy a threaded brass plumbing tee and screw in barbed adapters (or whatever adapters including compression-type fittings that work with tubing O.D.) of the size you need for the tubing size and type that works best for you.
Any good plumbing shop (not "big-box").
How do you attach it to the hot water line that you don't have? Just plumb it between the tank and the output pipe so all output goes through it - close to the tank would be easiest probably, but beware of freezing.
