Tipping point for water change volume in QT

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RuuToo

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So, let's say for argument's sake I'm an idiot that should have quarantined and didn't (like, so many times now, but this last time is just so bad), and I'm going to restart my approach to new additions.

I want a proper coral QT this time - we're talking 3 months in coral prison for new inmates. I don't have that much space, so I would like to run the absolute bare minimum of equipment

I jumped on the auto water change bandwagon a while ago, and I'm doing ~2 gallons per day that is currently going down the drain. Let's say I take that 2 gallons (or maybe up to 3), and direct it into a quarantine tank (that then dumps its overflow down the drain). Is there a max size of QT tank where that 2-3 gallons per day eliminates the need for additional dosing and auto top-off (the main tank is trident controlled and running pretty abundant and stable calcium/alkalinity so it should always be a high quality input)? I'd like to be able to just provide light, heat and flow to this tank and know that even difficult SPS could be kept there for an extended stay.

I've read Randy's articles on water changes, and the idea that water changes can eliminate the need for Ca/Alk dosing doesn't really work, *but* in this case I really could be doing a daily 25-50% water change (on a frag system that likely wouldn't have the same kind of demands as a big established reef).

So the question is this, basically - what size coral QT system could be sustained long term using the 2 (or up to 3) gallons per day that my tank is currently throwing down the drain? I was thinking something in the 10 gallon range would probably be feasible, but is that underestimating? Overestimating?

Thanks,

Dave
 
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mdb_talon

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I think you would be fine from an alk/calc perspective as long as you keeping the params stable in your DT it coming from.

However you mention wanting this to also eliminate the need for top-offs...which it certainly cannot do. Reusing salt water does not eliminate evap in that QT/frag tank so salinity will rise without adding freshwater no matter what.
 
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RuuToo

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I think you would be fine from an alk/calc perspective as long as you keeping the params stable in your DT it coming from.

However you mention wanting this to also eliminate the need for top-offs...which it certainly cannot do. Reusing salt water does not eliminate evap in that QT/frag tank so salinity will rise without adding freshwater no matter what.

Are you sure about that? That was my first thought too, but how exactly would salinity climb in a system with an influx of new constant-salinity water? I'm pretty sure that there is an equilibrium there too (just like there would be with the levels of everything else that might change over time)
 
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mdb_talon

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Are you sure about that? That was my first thought too, but how exactly would salinity climb in a system with an influx of new constant-salinity water? I'm pretty sure that there is an equilibrium there too (just like there would be with the levels of everything else that might change over time)

Yes there is zero doubt. If both start at 1.025 and everyday you flow 30% of volume through with 1.025 then you any evap is getting replaced with 1.025 water. If there is any evap(there will be) then salinity will rise in that scenario.

Looking at it another way lets say .5% of total QT volume evaps daily...you going to end up pushing though 30.5% volume with full salinity water. 30% of that just replaced existing water. The extra .5% is essentially topoff.
 
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I don’t think there’s any argument that the system will have slightly higher salinity - no doubt. What I’m not sure you are accounting for is the fact that there is ioutflow from this tank as well (unless it were evaporating at the same rate as inflow, which it would not be under any normal circumstance). If there is outflow of this elevated salinity water, and inflow of lower salinity water then there is probably some super complicated mathematics that would tell me what size tank I could hit before the difference in salinity became too great. Sadly, maths is not my strong suit, but I was idly hoping that someone with an advanced degree would be bored enough/interested enough to look at it :)

Dave
 

Randy Holmes-Farley

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So the question is this, basically - what size coral QT system could be sustained long term using the 2 (or up to 3) gallons per day that my tank is currently throwing down the drain? I was thinking something in the 10 gallon range would probably be feasible, but is that underestimating? Overestimating?

Thanks,

Dave

A few gallons, depending on the alk in the reef tank.

You will, of course, need to remove the same volume you add each day, and there will then be no salinity change if you add back RO/DI to replace evaporation. If no ro/DI is added, salinity will rise too much
 
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RuuToo

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I feel like either I'm explaining this wrong, or you guys are all wrong (or at least, not as right as you think you are...) about the salinity.

Bit of a thought experiment here...

Let's say I have a 5 gallon bucket, and I flow (for an absurd example here) 100 gallons of "normal" salinity saltwater per hour "through" it (I pour it in the top, it overflows down the drain). Am I eventually going to have a salinity issue in this bucket because of evaporation? Why not?

This QT tank would operate under the same principle, only on a smaller, slower scale. As water is slowly and continuously pumped into the QT, it will be overflowing down the drain. There's no doubt that evaporation will come into play and raise the salinity a little, but there's constant dilution incoming and an outflow of slightly higher salinity water to keep things in some sort of equilibrium - I'm just not sure where that equilibrium is.

To be honest, I'm going to do this just to see what happens, and I shall be keeping track of salinity every day to report back.

I just think that it's a nice way of getting additional use out of my water change water, and if it works it means that anything that goes through the process should be more or less perfectly "pre-acclimated" for the main display's water chemistry - personally I think that's a pretty big positive.
 
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RuuToo

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So, I set it up in a 5.5 gallon aquarium next to my main tank. It's running at 78 degrees, with significant flow and ~2 gallons per day being exchanged (so it's basically getting a 40% daily water change). It overflows into a nearby drain. Long story short, without any kind of lid the salinity stabilizes at 1.027 (yes, I am sure about that, I even stopped the water changes for a couple of days and let it evaporate to 1.028 then watched as the salinity went back to 1.027 when the water changes restarted). With a lid eliminating some of that evaporation I suspect that it will be close enough to 1.026 for it to be a non issue - that will be the next test.

I'm actually curious how it's going to hold up as a long-term tank for corals - will I be able to O.D. on reef roids / amino acids / whatever and let the water changes just deal with all of my excess nutrients while maintaining super high broadcast feeding levels? There's probably a really crazy build thread in here somewhere.
 
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RuuToo

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You did. It doesn’t. i did. I did. You were all wrong. It’s entertaining that you still think you’re right with such enthusiastic certainty. Please let Me know how long I need to wait before my salinity will start rising beyond the current equilibrium. I will more than happily keep you in the loop for as long as you need. I know that this isn’t particularly intuitive, but it is what it is I’m afraid.

Again though, am I in some way explaining this in some way that isn’t clear enough? I have to assume it’s something like that since I know many of you are pretty smart people.
 
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I want to flow my automated water change water through the QT tank as it heads to the drain, thus giving my water change waste water a “second function” for free, and giving my QT system 40% daily water changes to eliminate a lot of maintenance there.

Those are both good things, and I am unreasonably excited about this.
 

Randy Holmes-Farley

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You were all wrong. It’s entertaining that you still think you’re right with such enthusiastic certainty.

Perhaps you are explaining poorly, but what is written will cause a rise in salinity.

Salinity will rise if you never add any fresh water to replace evaporated water. There is no other way possible if the only water additions are 35 ppt seawater. Yes, you overflow seawater out of the tank, and add some back, but you are adding more 35 ppt water each day than is removed (because of evaporation).

If you are adding 35 ppt seawater, overflowing QT water to the drain,a nd never adding fresh water to replace evaporation, then if you tell me the exact evaporation rate in % per day, and the rate of water added (% per day), then I can tell you exactly how much the salinity must rise each day.

Let's take an example.

3% evaporation each day, and 40% of the tank volume added as 35 ppt seawater (40%) and then the tank water is overflowed (37%) at whatever the salinity is at the time.

Day Salinity
1.0 35.0
2.0 35.6
3.0 36.1
4.0 36.3
5.0 36.5
6.0 36.5
7.0 36.6
8.0 36.6
9.0 36.7
10.0 36.7
11.0 36.7
12.0 36.7
13.0 36.7
14.0 36.7
15.0 36.7
16.0 36.7
17.0 36.7
18.0 36.7
19.0 36.7
20.0 36.7
21.0 36.7
22.0 36.7
23.0 36.7
24.0 36.7
25.0 36.7
26.0 36.7
27.0 36.7
28.0 36.7
29.0 36.7
 
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RuuToo

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There's no doubt that evaporation will come into play and raise the salinity a little, but there's constant dilution incoming and an outflow of slightly higher salinity water to keep things in some sort of equilibrium - I'm just not sure where that equilibrium is.

I don’t think there’s any argument that the system will have slightly higher salinity - no doubt.

I mean, *I* felt that I was clear earlier on that I knew that the salinity would increase :)

I think my issue is with this...

If no ro/DI is added, salinity will rise too much

The salinity seems to have stabilized at 36ppt with my current setup (indicating I guess somewhat less than 3% evap per day - I'm a geneticist not a mathematician, sums are hard). I wouldn't say it's ideal, but if I partially lid this thing up and reduce evaporation I can probably get it close enough to 35 (whether limiting gas exchange like this is a good idea is a whole other question). I suspect that some people may have read this as "I want to top off with salt water, and I don't think that this will raise my salinity", which isn't really the question, or what's happening here - It's more like defeating the need for top-off with water changes, which is obviously a bad idea unless you have a really small tank and a significant daily exchange volume.

I do wish I had a refractometer that would measure with more fine-grained units though, and my apex conductivity probe is sort of "advisory at best"
 
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