Auto Water Change - Pros and Cons of Completing AWC in 2 Hours vs. 24 Hours

I just read through the Automatic Water Change instructions in the setup guide, and I actually understand them.

The suggested program has the whole daily process completed in less than 2 hours.

I was wondering: Why wouldn't I stretch this out over the course of 24 hours?

My thinking is that making smaller changes over the course of a longer period of time would result in much less variation in my sump water level, with consequently in much less likelihood that my ATO system would be activated. I also imagine (though it's pure speculation) that there would be less stress on the pumps running at a lower speed over a longer period of time.

Any thoughts?

I agree with you. I have my AWC change 20 gallons over the course of a week working 24 hours a day... I'm lucky because my DOS is in my basement and don't hear it. Some people might find the whine it makes annoying when it is quiet in the house so maybe they'd want to have it run over certain hours.

I have my DOS set up to change 1.5 gallons of water per day, and I use the "drain first then refill" approach. At 9:00am it begins draining out 1.5 gallons until 10:30am. At 10:30am, it then starts refilling 1.5 gallons until noon. During the whole process, I have my ATO disabled from 9am - noon so it doesn't try to refill with fresh water. The water level in my return chamber drops by about 3" but not enough to cause problems with my return pump, and the skimmer chamber level does not change. When the ATO finally resumes operation at noon, it typically runs for about 1.5 minutes to catch up on evaporation loss.

I've had this set up and running daily for 4 months now, and it's been working flawlessly. I can't yet comment on DOS pump head wear & tear, but I have a couple of spare pump heads on hand if/when they start having problems.

Unless you do drain then refill, slower is better. It allows changes to occur more gradually, which corals prefer and is less likely to be affected by ATO. I would consider pushing it out to 6 days a week 24 hours a day and refill on the 7th day for the most consistency.

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The sound level is something to consider. I haven't installed the DOS units yet so I have no idea. I do know that my wife is ultra-sensitive to sound, so I'll be watching out for that.

My DOS is mounted on the wall in the garage where I have the mixing station set up. The aquarium is on the opposite side of that same wall in my living room, and I can definitely hear when the DOS is running. It would be quieter running at a slower speed, so if that's a big issue then slower is better. Where will your DOS be mounted?

I was internally arguing with myself about the drain-then-refill possibility.

I was thinking that the simultaneous drain-and-refill method, displayed in the suggested DOS programing in the setup guide, wouldn't give as precise a percentage water change as might be expected because, with the see-sawing effect, the old saltwater being removed would not be "pure" old salt water but rather "diluted" by the new salt water. Then I realized that in the context of a 120 gallon tank (plus sump volume) the difference between the two methods is likely to be insignificant. There might be more of a significance if I were dealing with a nano setup.

Given that thinking, I'm more inclined to go the simultaneous route so that my water levels remain relatively undisturbed.

I like your schedule. What total percentage of tank volume do you aim for over the course of your 6 days? 10%?

I am nicely set up for this, with a large plastic reservoir from US Plastics (well over 40 gallons) that would let this process run for many days.

With the AWC in place, do you (or anybody else) consider it advisable (or inadvisable) to do larger water changes periodically?

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Originally Posted by rtolz

I was internally arguing with myself about the drain-then-refill possibility.

I was thinking that the simultaneous drain-and-refill method, displayed in the suggested DOS programing in the setup guide, wouldn't give as precise a percentage water change as might be expected because, with the see-sawing effect, the old saltwater being removed would not be "pure" old salt water but rather "diluted" by the new salt water. Then I realized that in the context of a 120 gallon tank (plus sump volume) the difference between the two methods is likely to be insignificant.

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You are correct - the dilution factor is so small, it is insignificant. BTW, the AWC example in the DŌS manual is just that - an example. It's not necessarily a recommendation. On the other hand, the DŌS Get Started Guide (https://www.neptunesystems.com/getstarted/dos/) illustrates how to do continuous AWC over 24 hours, and this is the recommended approach for most.

MY DOS will be mounted in the "laundry room" on the other side of the wall adjoining the tank. The tank is in the living/dining/kitchen area where we spend the majority of our waking time, so I don't want lots of added noises.

Is there any difference between the DOS's sound levels depending on how many ML/MIN it's pushing?

I already have a buzzy circulation pump on an Aqua-Maxx denitrator that I have to figure out how to silence or replace, and I don't want to add any more noise during waking hours.

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Originally Posted by rtolz

I was internally arguing with myself about the drain-then-refill possibility.

I was thinking that the simultaneous drain-and-refill method, displayed in the suggested DOS programing in the setup guide, wouldn't give as precise a percentage water change as might be expected because, with the see-sawing effect, the old saltwater being removed would not be "pure" old salt water but rather "diluted" by the new salt water. Then I realized that in the context of a 120 gallon tank (plus sump volume) the difference between the two methods is likely to be insignificant. There might be more of a significance if I were dealing with a nano setup.

Given that thinking, I'm more inclined to go the simultaneous route so that my water levels remain relatively undisturbed.

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A 30% continuous change is exactly equivalent to a 26% drain then fill water change from a nutrient export or addition standpoint. You trade a small amount of increased salt usage for a huge increase in consistency. RHF has a good write up that explains the intricate details.

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This answered my question thank you I was worried that the water i was putting in was been taken out right away !! this is a great idea but what's the % of fresh water from the day before being removed from the next days water change ?

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Originally Posted by SuncrestReef

I have my DOS set up to change 1.5 gallons of water per day, and I use the "drain first then refill" approach. At 9:00am it begins draining out 1.5 gallons until 10:30am. At 10:30am, it then starts refilling 1.5 gallons until noon. During the whole process, I have my ATO disabled from 9am - noon so it doesn't try to refill with fresh water. The water level in my return chamber drops by about 3" but not enough to cause problems with my return pump, and the skimmer chamber level does not change. When the ATO finally resumes operation at noon, it typically runs for about 1.5 minutes to catch up on evaporation loss.

I've had this set up and running daily for 4 months now, and it's been working flawlessly. I can't yet comment on DOS pump head wear & tear, but I have a couple of spare pump heads on hand if/when they start having problems.

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Originally Posted by mazureka

This answered my question thank you I was worried that the water i was putting in was been taken out right away !! this is a great idea but what's the % of fresh water from the day before being removed from the next days water change ?

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My Reefer XL 425 (minus rock, sand, and equipment) has about 105 gallons of water. My daily auto water change replaces 1.5 gallons, or about 1.43% total volume. The next day, the amount of newest water swapped out would be about 0.02% (1.43% x 1.43% = 0.0204%). In other words, it's not even worth worrying about the amount of new water being removed because it is so diluted with the other tank water.

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Originally Posted by mazureka

This answered my question thank you I was worried that the water i was putting in was been taken out right away !! this is a great idea but what's the % of fresh water from the day before being removed from the next days water change ?

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The same percentage that was changed. This is how the whole thing stacks out in a 1 month timespan with the same amount (20%) of water used.

Monthly 20% WC - 20% of water replaced
Weekly 5% WC - 18.5% of water replaced
Daily 2/3% WC - 18.2% of water replaced
Continuous WC - 18.1% of water replaced.

As you can see clearly, the difference in initial water remaining has almost zero difference once you go from weekly down to daily or continuous. However the convenience factor of filling a bucket and forgetting about it, and the technical ease of not having to worry about water level changing makes the ideal choice obvious. Others may think its wasteful, but you own a saltwater tank, so we are talking pennies a month of extra salt on a tank that probably goes through that in an hour of electric.

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need help so i went to prime my lines so when i push the button on the unit or turn it on in apex the head spins the wrong way !! what the hell i hooked it up right but the head spins the wrongs why what should i do ??

can i get the code for the ato to be off

I have a apex atk so would the code be the same ?

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Originally Posted by mazureka

need help so i went to prime my lines so when i push the button on the unit or turn it on in apex the head spins the wrong way !! what the hell i hooked it up right but the head spins the wrongs why what should i do ??

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The prime buttons only work in the direction of the arrows. If you want them to function properly, both heads need to be set to add and tubing arranged so the prime adds water for the NSW line and sucks tank water from the tank for the OSW line.

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Originally Posted by mazureka

can i get the code for the ato to be off

I have a apex atk so would the code be the same ?

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ATO does not need to be disabled when doing continuous water change. In that control form, you should disable the AWC when the higher level optical closes (add If Switchname CLOSED Then OFF to advanced tab of both DOS heads). You should also stop AWC for high cond, low cond, and a low sump switch or optical if you have one.

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I'm doing the remove then add approach i think i got it but it waits till the next hour to fill the tank with ro water which is fine

I have a 220 tank and am thinking about setting up the AWC for 20% or 44 gallons every 2 weeks. do i need to keep the salt water circulation and heated that i am adding to the tank? Sorry for a simple questions just want to make sure that i do not mess anything up for a simple Question

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Originally Posted by wdiryan

I have a 220 tank and am thinking about setting up the AWC for 20% or 44 gallons every 2 weeks. do i need to keep the salt water circulation and heated that i am adding to the tank? Sorry for a simple questions just want to make sure that i do not mess anything up for a simple Question

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It depends on how quickly the water is changed.

If you are doing continuous, no heating or circulation is necessary and is actually detrimental because heating causes some precipitation.

If you are doing remove then add as quickly as the DOS will go, its helpful to heat the water at least a little bit (like 75).


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