Mr. Saltwater Tank

Do You Really Need an RO/DI Unit?

One the basic laws of keeping a saltwater tank is to only use water from an RO/DI system for your tank. RO/DI water is very, very clean, so by using it, you won’t be introducing bad things into your system such as silicates, heavy metals and chorine that can cause a major of problems in your tank.

(If you are interested in learning the chemistry behind how an RO/DI unit works, read this article)

RO/DI water is usually available at local fish stores so a lot of saltwater tank owners ask,”Do I need my own unit when I can just buy it at the fish store?”

For all the tank personalities, my answer is always, “YES!” and here’s why:

1. Its a matter of having insurance.

If anything ever goes wrong with your tank such as it starts leaking, or you need to do a large water change, by having your own RO/DI unit, you’ll have the piece of mind that you can make all the water you need, when you need it. If you didn’t have a RO/DI unit, then you’d either have to store large amounts of water in jugs which takes up a lot of space and are heavy and not fun to move around. And, if you didn’t have your own unit, then you’d have to bank on the fact that either your buddy who has a RO/DI unit would want to take your call when you call them AND that they’d be willing to have you come over at any moment to pickup more water. Oh yeah, and you’d have to rely on the fact that they actually had enough water on hand to meet your needs at the time. Murphy’s Law would apply here that chances are when you need the RO/DI water, your buddy would have just used up his supply so you’d be fresh out of luck.

Also, I’ve yet to see a local fish store that is open 24/7 so if you needed the water when are closed (Murphy’s Law again), well, I hope you (and your tank) can survive the wait.

There is a certain peace of mind knowing that if anything goes wrong in your tank, all the water you need is within your reach.

2. Its actually cheaper in the long run

Most local fish stores sell RO/DI for 25 cents a gallon which is pretty darn cheap. But what most tank owners don’t think about is the amount of time they spend driving to the local fish store, filling up their jugs, loading their jugs (did I mention they are heavy?), driving back home and then unloading your jugs. And unless you have a set routine to get more water from the fish store, chances are you’d run out when the store is closed.

Doesn’t sound like a big deal to you? Well here’s where you really save money:

By not going to the local fish store, you aren’t walking by all the pretty display tanks and being tempted to buy that fish or coral that is just calling your name! (Remember those puppy stores in the malls back in the 80’s? There is a reason they put them by the front entrance!)

I actually think local fish stores should make their RO/DI water 10 cents so people couldn’t resist coming into the store. More people in the store = more sales. How many people do you know who go into the fish store to buy one item and come out carrying 3 or 4?

When you put a RO/DI unit in your house, you remove temptation and retain money.

3. You get to maintain control

When you own your own RO/DI unit, you’ll know exactly how long ago the filters were changed and how clean the water coming out of it really is. No more counting on someone else to keep up their system, you’ll know when its time to change the filters and how clean the water is coming out of the system.

Even if you are a tank dabbler, you can pickup a 35 gallon/day unit that doesn’t take up a lot of room and will keep you from spending a lot of time dealing with your tank.

A good RO/DI unit, isn’t cheap, but they more than pay for themselves in less tank headaches and having everything you need on hand in an emergency.

Browse the Store! Questions?

Comments for this article (38)

  • Matthew Brady says:

    Going to use this article to convince my parents to buy a RO unit
    for My 55 gallon future Reef tank

  • Matthew says:

    Do I need a ro/di if I’m doing a fish only tank?

  • I would still recommend it Matthew

  • Ash Maxwell says:

    I looked on Amazon for the unit, they are expensive as hell! Are they really necessary or is there a cheaper place to get one?

  • Ash…a good quality RODI unit can be had for $120 ish USD. That’s a small price to pay given how much will be invested in a good reef system

  • Ash Maxwell says:

    I live in the UK though, they are much different over here.

  • Ash Maxwell says:

    Could you do a video on how and why you need to use an RO/DI filter unit?

  • Kyle says:

    Having your own unit saves money over the long haul. I can’t imagine why anyone would want to haul water. Get some aging barrels and a pump and you’re set.

  • MIke says:

    I got an RO filter and did not realize it was did not include a DI, that still ok for a marine tank?

  • Daniel says:

    What would you consider to be the most top of the line RODI system , I’m talking for extreme hobbyist , best features, cleanest water ex.

  • Derek says:

    I’m have a 125 saltwater set up do it need to drain all the water and start fresh with ro-di water or can it just start doing water changes with the ro-di water have corals and fish in my tank now .

  • Duke Sweden says:

    If having an RO/DI unit is insurance against ever running out of water, why would your “buddy” ever run out of water just when you need to get some from him?

  • Duke…they might need a lot of water sudden for an emergency water change, their filters might go bad and they don’t keep a replacement on hand, they need water and forgot to refill their holding containers, etc…

  • Duke Sweden says:

    I know. I was just playing devil’s advocate 😉

    Thanks for doing what you do. It’s gotta be a lot of work, on top of your money gig. It’s much appreciated.

  • Connor says:

    I have a 300 gallon tank that I’m going to set up, can I use tap water to enitially fill up my tank to get it started and treat the water? I have a RO/DI system but it would take days to fill this tank up just running that.. I would from then on out use my RO DI system for water changes and top off

  • Duke Sweden says:

    Connor, I can imagine how long it will take to generate 300 gallons of RO/DI water, and I sure wouldn’t tell you to just buy 300 gallons at your local LFS.

    What I will suggest, however, is that you load your sump with enough Chemi Pure for your system. At the very least it will remove silicates and other minerals from the water which will at least give you a fighting chance against the almost certain outbreak of brown algae (diatoms) that you will experience from tap water. My opinion anyway. And that’s only if you see no alternative to using tap water at first.

  • Connor…I fill all my new tanks up the same way you described: Run the output of the RODI unit into your tank and let it rip! Here’s another time a 150 gpd RODI upgrade makes sense as your unit will make water twice as fast. Plus you’ll cut your waste in half.

    You could probably get away with starting your tank with tap water and dechlorinating it. For me, I look at the amount of money I’ve invested in the system and the added time waiting for the RODI unit to fill the tank is a drop in the bucket.

  • Adam says:

    I got a RO unit but without the DI – how important do you think the DI part of it is? (i’m running a 75g Fowlr tank with a sump/skimmer/chemipure)

  • Adam…the water coming out of the RO unit is probably low TDS, usually around 2. 2 vs 0 doesn’t sound like a lot but it can build up over time.

  • Rahul says:

    As soon as i started making my ro/di water my water bill was being raised about $40 a month and i was only making about 30 gallons a month…. does anyone else have an issue similar to me? im wondering if i should just buy it from my local fish store 50 cents per gallon

  • Marc says:

    Rahul…good point. Making RODI water means you have to strip out the non-water elements, and the only way to do so (efficiently?) in a home is to use a fair amount of water to “flush away” the stuff you don’t want. In most home RODI setups you will end up using anywhere from 200%-300% more water than compared to just pouring tap water in to your tank. Not at all insignifcant amounts, especially for larger tanks.

    The two main things you can do are:

    1) Change the Wastewater Ratio: some systems will allow you to choose between different waste-to-product ratios by changing out the flow restrictor line. There is a trade-off, though. While using a more restrictive line will produce less wastewater, it will also reduce the lifespan of the RO membrane because you’re forcing a higher % of water through it and therefore the membrame will clog up sooner.

    2) Use the Wastewater: the wastewater is fine for watering plants, flushing toilets, and probably a few other things. (Do NOT use where the increased sediment content can cause an issue, ie dishwashers, water heaters, pools, sprinklers, drip lines, washing your car.) The main thing most people will use it for is watering plants or grass.

    The best combination of these two from both cost and conservation standpoints, if its feasible in your scenario, is to use the more open restrictor (more wastewater, longer membrane lifespan) and to use the wastewater where you can. If space is not an issue, inexpensive 55g blue food barrels can be found online for as little as $15. If esthetics is an issue, nicer looking (and more expensive) barrels can be purchased. I use barrels that look like wine casks.

    For a few more bucks, I leave a small, inexpensive aquarium pump and attached hose in the barrel and use an electric foot pedal switch for the pump. Makes it very convenient to quickly fill a watering can.

  • Tim says:

    Hey Marc, I’m looking into buying an RO-DI system and the first thing that comes to mind is will these systems strip away the essentials needed in my water for my corals such as calcium etc,? The current water filter I’m using works fine but the algae outbreaks are just driving me nuts! Thanks, Tim

  • David says:

    I disconnect my RODI unit and store it in between water changes. When I reconnect it I flush it for five minutes. Does this impact the viability of the filtration?

  • David…how do you flush it?

  • antonio miami says:

    I just wanted to say thank you for being there when I needed advice or when I needed to refresh my memory I been watching your shows for years .thank u mr. saltwater I really do appreciate it sir

  • Sean Benson says:

    I’m curious. Since the PH of RO/DI is much lower that we keep out tanks is it a negative to run the RO output directly to the sump for evaporation make up? My 110 gallon system evaporates around 5 gallons a day. I understand the risk of directly plumbing to sump.
    I ask because I can’t keep ph above 8.01 while maintaining alk of 11.3 460 CA and 1500 mag. I’ve wondered if having a resoviour and adding a touch of kale would rectify. Any thoughts are appreciated.

  • Sean Benson says:

    Meant much lower PH*

  • Sean Benson says:

    Haha. I need to proof better. Also….adding a touch of KALK

  • MeerZameer says:

    Thanq Sean for putting up the question I was googling for…

    It would be very helpful if anyone can enlighten me on the evaporation make up

    Thanks in advance

  • MeerZameer says:

    I am about to start a 150g tank…. And would like to make everything extremely set before the start.

    My tank crashed twice in the last year and now I desperately want it to survive this time.

  • Lou says:

    I only use harvested rain water for my reefs and propagate everything from softies to more demanding pavonia and other hard corals. I have used rain water for the last few years and never had any crashes or otherwise. What works for one may not for others but that said, I’ve seen strict users of RO/DI still experience system crashes!

  • google says:

    Thanks for finally talking about >Do You Really Need an RO/DI Unit?

    – Mr. Saltwater Tank <Loved it!

  • David House says:


    My question is, the water coming out of my RO/DI is a lot colder than what my tank water should be. Adding small amounts at a time would this mater or should I heat up the water until it is the correct temp?

  • Gabriel says:

    Can I get purified drinking water then Declorninated with sechem prime and then add the salt and everything

  • Randy says:

    New to this hobby, so r/o system is hooked up under your kitchen sink correct.? And if this is the case then u still need to fill up containers to put in tank correct ?

  • stephany r says:

    They are convenient until I received my $3000 water bill. Awesome.

  • Earl says:

    If I have an RODI system, do I need a filter? or should I still get a sump as well.

  • JanC says:

    You still need a sump because even though the water going in the tank is pure, waste will accumulate in the water over time – that is why we do water changes and use sumps or filters. . You probably could use a filter instead of a sump especially if you have a small aquarium but having had freshwater tanks with HOB filters for years I HATE THEM – the intake is constantly getting plugged up by vegetation, snails etc and they are a pain to clean – I have one tank I have to clean the intake tube several times a day to keep it clear. And the fish are such waste producers I have a quarter inch of sludge in the bottom of my filer about every 3 days – so that’s how often I have to clean it and it’s a top of the line HOB. I would consider a canister filter but the intake tube would still get clogged and they are more difficult to clean. Seriously thinking about using RO/DI water for my freshwater tanks but the system filters out EVERTHING and that’s fine with a saltwater tank because the saltwater you add has the trace elements that you need – there isn’t such a thing for freshwater tanks so I’d be afraid the fish and plants would be seriously lacking in some important nutrients. Has anybody used RO/DI water for a freshwater tank? The one nice thing it would do would be to reduce my PH (my tap water has a PH of 9.4 and I have to use a reducer to change it to 7.0 before I add it to the tank (I can’t pour PH 9.4 on the heads of my fish and then adjust it – I learned that the hard way after losing half my fish – so each bucket of water added is PH adjusted and conditioned before it goes in my tank (making it impossible to use a Python to do water changes since there is no way to add chemicals to your tank as the water flows down the tube – with the Python you can only add it before or after the water change – which won’t work with my PH situation. So yes you need a sump or a filter – not both

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