Terrible Advice Tuesdays (T.A.Tues): The Tank That Never Needs To Be Topped Off

Terrible Advice Tuesdays: To avoid setting up a top off reservoir or having to refill your top off reservoir, just plumb the output line from your RODI unit straight into your sump. Then install a float switch shut off valve to turn on/off the water flow as the water evaporates out of your tank.

The rest of the story: Please don’t EVER do this…EVER. The reason is that if the shut off valve fails then your RODI unit will continuously make water and empty the water into your tank. Your sump will fill with water and then overflow all over your floor.

Oh and don’t forget that you’ll be continuously filling your tank with freshwater, so your saltwater tank will become a freshwater tank, killing everything inside.

The safest way to set up your auto top off unit is for your RODI unit to be plumbed into a holding container that is separate from your tank AND separate from your top off container. This will prevent your tank from continuously having freshwater pumped into it in the event of a float valve or switch failure.

Again, please don’t ever plumb your RODI until straight into your tank or top off reservoir. Just last week I got an email from a viewer who made this mistake and he came home to flooded house.


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Comments for this article (27)

  • Dave Castillo says:

    Ive actually done this,kinda. I used my 55 gallon drum of RO/DI water for my ATO reservoir. Float valve malfunctioned and put 55 gallons of water into my sump/tank and floor. Salinity went down to about .014 or so. It killed my cleaner and coral banded shrimp and a starfish. Luckily everything else lived. Took me a week or so to slowly raise salinity back to normal. Regardless to say I dont have my ATO plumbed to my RO/DI reservoir…

    Most lessons are learned the hard way.

  • Mike says:

    Wow.. I was just about to do this!! hah!

  • Angel says:

    Stupid is what Stupid does.
    A rule of thumb.. Read on the equipment you are about to pull in.
    True there is trial and error in everything you do, however “Saltwater” is
    an expensive hobby, so keeping to error at a bare minimum is the trick of the game.

  • Jerry says:

    It’s also bad to allow your rodi to top off anything in short bursts, due to evaporation. Those short runs don’t purify the water as well. The best method is to have a float valve lower in the reservoir that kicks the rodi on, with the top shutting it off

  • hectorevalero says:

    Oops mine is plumbed this way, I’m just lucky to work from home so I monitor daily but will immediately setup a holding container

  • Barbara says:

    Mark would never have to put up these tips or even have this site if no one ever made a mistake or could figure out everything from package instruction (which some do not even have). This is a very popular site so I guess we all must be to some degree “stupid”. I for one appreciate people telling of their bad experiences and I hope they continue without fear of being called stupid.

  • Mac. says:

    My RO unit has an auto shut off when full and is plumed into my 20 gal, top off and 50 gal water change drum. Been that way for years..No problems…
    1. I have two floats one above the other..Second one acts as a safety.
    2. I have to physically turn on the RO to fill the Top off tank….

    When it is on I were a large wooden Key around my neck till I shut it off..When you’re 65. You need reminders

  • taylor overeem says:

    It seems to me if people want machines to do everything where is the fun in taking care of your tank also I rather fill the sump up every day and dose manually that way you can only blame yourself

  • Jon says:

    I totally agree with the previous thread. Yes indeed!

  • Tony says:

    My ATO a 5 gallon bucket next to my Refugium. My refugium is a 25 gallon tank. With only about 14 gallons of saltwater. I only fill the 5 gallon bucket as needed with RODI WATER. so if something fails I am In kind of safe zone. In my garage I have a 5 gallon bucket with a Float switch/valve that fills up, and it’s ready to fill up my ATO Bucket by the old school way walk the bucket the the ATO and fill it up. I was thinking of doing just that at one time but something said DON’T DO IT. LUCKY ME.

  • Luke says:

    I actually did make this mistake (first reef tank). My RODI automatically tops off my sump. However, I did put an overflow in the sump which would carry any “floodwater” down into the crawlspace… even thought about plumbing it into the sewer line. This doesn’t, however, solve the problem of lowered salinity in case of float malfunction, it just prevents the flooded living room. I will probably look into getting a storage tank as Mark suggested.

  • Jon M. says:

    Yes doing everything yourself would be great but for people like me who are disabled but still LOVE this hobby, having things self automated makes this hobby possible for me. Yes I have more money in my 70g than I would if I wasn’t disabled but it’s what it takes to make sure my tank is happy when I’m in the hospital for a week 5 times a year. Apex is my friend lol. I’m just glad I have my beautiful tank and be able to because of mechanics 🙂 This is my first post but I read every post put up on this Site, keep up the great work Mark I love reading your post and some day I will be able to afford a set of your book to read while in the hospital.

  • Cristian says:

    I thought of running the RO/DI directly into my sump, but my common sense said there is a chance for error so I designed a refugium with a 5gal divider which I fill up every few days for the top off…

    Ive also added the leak detection module to my Apex which alerts me in-case there is a leak (has happened when walking away from the RO/DI while filling the ATO container to play with the baby)
    Also I’ve since added a solenoid valve, controller by Apex, to the RO/DI and it shuts off the flow if the Apex leak add-on detects a leak or the float switch gets tripped… can never have enough safety.

  • Tony says:

    I personally like the the simple methods less gizmos, counting on electronic devices to do everything, not safe in my book. Has your toilet tank ever overflowed probably never, thats just a float ball a flap and chain. The simple the better.

  • Matt says:

    Hi I’m Matt and I’m in my mid thirties… I have recently come to realise thanks to my (unbeknownst to him) new friend Mac that I need a large wooden key to hand around my neck… I just wish I was joking!

    Nice trick Mac… Should be a tip all on its own for the majority of us at some point or another! 😉

  • Pierre - Yves Bouic says:

    I think that everyone should create a false power outage on their set-up especially after new equipment is installed of this type. Its amazing how many people have problems with self syphoning or other shut off types of equipment, especially when DIY is their middle name. Lots of people don’t even know what the min/max water level should be in their sump as when the power is shut off and when plumbing all drains back to it, the holding capacity of the sump is not enough and whoops water goes where it shouldn’t. Good tip mark.

  • Dale says:

    Yes, that was me.
    I guess learning the hard way is tough love. To make matters worse I had it set up properly prior to having this great idea of not having to fill my top up container continuously. My loss was huge with a full tank of corals, critters and fish.
    Just when I thought I had things figured out.

    Thanks Mark for all your good advise!

    Only wish I had read this before going on vacation.

  • Dale says:

    Oh, yes Angel I do feel stupid and have beaten myself up over this for weeks now.
    This has been my only hobby for the past 7 years and has helped me get through a very tough time in my life.
    Its comments like yours that turn people off from doing any hobbies because they don’t want to be called stupid.
    Filters are a great thing, some people should learn how to use them.

  • Love your videos they look very professional and flow well. Do you do all of the editing yourself?

  • Dan says:

    Can someone post simple schematic of the ideal (meaning safe and easy) auto-top off system, preferably using only mechanical floats…

  • Lou…I edit all my videos myself

  • Gusher says:

    Hi Mark. I admire your work and advice. I’m new to the hobby this being my first tank so I follow your tips. I’ve even tried to set up my tank just like yours but on a smaller 70g scale lol even put my Radion on once you said they where good and you went total led lol. My question is, do you have a list of the gear your using so I know mine is done right? I’m using everything I can see on yours but don’t know everything that’s under lol, just want to do things right and be like Mark lol 🙂

    Thanks Jon M.

  • David says:

    I have a 10 gallon tank next to my Sump/Fuge. I have a RO/DI float valve (from bulkreefsupply) on it, and my RODI water goes in the 10 gallon tank. I also have the pump for the kalk reactor in the RODI tank. I have a Tunze Osmolator to take the RODI from the 10gallon to the Sump. SO far has worked well!

  • NavyCove says:

    Yup, been there, done that. I have done every one of the things above. I think I now have it foolproof. Unfortunately I also have to make it idiot proof. I’m headed to the shop now to make a large wooden key. Great idea.

  • Patrick says:

    Hello all,
    I myself have a setup similar to DAVID. I have a 120 with 33 long sump us gone bean diy overflow. My RODI runs to a 15 gal container next to me tank/sump and a tunZee osmolator handles the top off. Works beautifully as long as you keep up maintenance on te tunZee virtual eye and the back up float switch. Love your videos mark. Look forward to them all the time. Anybody wanna do any frag swapping?
    Thanks guys

  • kevin says:

    Imho you can do this. However you want at least 4 safety measures in place.

    1. Maximum time the RO/DI unit runs
    2. Overflow switches (as normal)
    3. Spare overflow switches.
    4. Safety shutoff just in case the normal one fails.

    Imho it’s a good thing to look once in a while and fill the bucket or whatever you are using. So mostly i agree with mark don’t do it. It costs too much to be safe and it’s an easy job to refill the reservoir.

  • António Vitor says:

    I actually did this no problema for over 6 years…. But with some smart modifications…
    First I use relay switch and only pass trhough the switches 5v and a tiny current not evrn 50 mA… I use two electric switches for redundancies…. But the smart move was to put a electronic or mechanical temporisator… Only watch the level 2 times per day at 5 min its enough… If not enough I would increase the time just to be enough… If the timer clock fails no problem if a switch fails… No water…. If the two fails and let pass the current no problem only if you go out for 2 weeks… But after several years the float switches are brand new because if the weak current

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