sfsuphysics
Supporting Member
Please feel free to add to this, I'm a bit of a geek in that when I drive I space out (I know, bad) on the freeway and often thing of things like mathematically how something works, well one thing that I was thinking of was keeping up your calcium and alkalinity and magnesium and... well just anything else via water changes.
Bottom line is that will never work!* (* with some exceptions)
Abstract
If you get one of these "good" salt mixes that have the "proper" levels of this and that, and those are what you want, you will never have those levels in your tank with out dosing. If you have creatures that consume those elements, then you'll have less than what you started with, if you change the water then you're mixing a solution of less than ideal with ideal, the end result will be somewhere in between those two values since you're diluting your ideal concentration with something less potent and the result is always going to be less than ideal (just varying levels of how much less).
The test
So what I did was use excel to compute how much of a percentage of "it" you'll have left with various levels of water changes. The way it was set up was as a percentage of what we want, not some ppm or something, so it doesn't matter how large or small your system it works the same.
Assumption 1: Constant consumption of elements. Now I realize this isn't true, if there's 200ppm calcium vs 400ppm I'm guessing the uptake might be a little slower since the water isn't as concentrated. I don't pretend to know exactly how the creatures in our tank use them, my guess is that they'd simply stop consuming all together. I'm not trying to show you exactly how much you'll be left with, the purpose is to show you that you will never get back those initial conditions on day 1. The rate at which you lose will differ but again that's not really important, so take the actual percentages that are left with a grain of salt (or a whole bucket worth for later days!
)
Assumption 2: These are elements/values work in a linear fashion, so 400ppm/8dkH they all work in that same fashion in that to have half as much of the value, you take half as much of what that value is talking about. Things like pH don't work at all like that, the what needs to happen to go from a pH of 8 to 9 is much different than from a pH of 7 to 8. Even though the consumption doesn't work in a linear fashion, because the elements themselves do when they get doubled/halved/etc the idea still is valid
Assumption 3: I set my consumption rate to about 5% per day of the element, whether or not this is a true reflection of consumption again is irrelevant, if your consumption rate is slower then it will simply take longer to reach a lower value. 5% however does reflect a calcium level of 400ppm and losing 20ppm per day, so it's not totally off. Again with #1 I'm sure this rate isn't constant, but that doesn't matter.
What I did was take 6 separate cases. First case was the worst case, and no water changes at all, then a series of weekly water changes at 10, 25 & 50% respectively, a 50% monthly and a 10% daily which is used to reflect a "continuous" water change that some people are using (I did 10% a day just to show what will happen, the rate at which you do a continuous water change will affect how high or low it will get). I did this over a period of 60 days since that'd be equal to 2 monthly changes.
The results - Analysis
No changes: No big surprise here, you don't change the water your values will drop as elements are used up. Remember more than just corals consume calcium too! Most likely what would happen in reality is any coraline algae you have will simply stop growing, maybe even dying back and new stuff won't grow back in it's place. Snails and other inverts that use it for an exoskeleton will most likely have issues as well as it can't make a new skin properly. The graph will eventually level out, however these will be at levels too low to be useful in a tank, anyway you slice it this is not the way to go.
Weekly changes: Again no big surprise it has a dip that's similar to the first 7 days of the no change, then there's an infusion of new water which boosts it up slightly, but you'll notice never back to 100%, even with the 50% weekly changes (which is a lot of water to be changing) after a few weeks it never gets more than 80% of the normal. As time goes on the rate at which it is used slows down, but regardless of how slow it goes it still is going to be less than the previous week, they do eventually get asymptotic however and bounce around an average, that average even at 50% is roughly 70% of what you started with.
Monthly changes: This in behaves just like a weekly change except the infusion has a much more profound effect in how much it will change. While the monthly mark brings the values higher than that of some weekly changes, you have to realize the average is still below the weekly.
Daily change: This was to reflect the continuous change, due to my resolution being 1 day, it does a fairly good job of it and as you can see has an asymptotic limit that occurs quite quickly as such you don't see the up and down infusions of the weekly & monthly changes. When reality pokes its ugly head in the math and you don't get a linear usage of elements, i.e. calcium consumption slows down, you would see those spikes with a daily usage, however the end result is still the change, if you put in X concentration in to a mixture of X-Y concentration your end result is less than X. That being said this one would probably be the most preferable if you were to use only water changes to keep your elements up, it won't keep them up where you want them, but they'll stay fairly high regardless. Just note that's a lot of water, 10% daily over 60 days will be 6x as much water used to change as the volume of the tank! (That doesn't even taken into account using an RO unit and the waste water too).
The Graph
The Exceptions
Early on I said there were some exceptions to the rule here they are.
1: 100% water change. Yup if you pull all the water out, and put in fresh water with every water change you will in fact have 100% of all the values you start with... at least until the creatures start sucking them down! However this is usually not recommended to do except in the most extreme of catastrophic circumstances with a tank. Also note, this only works if you first pull all the water out, simply putting water in while you pull out dilutes your mixture.
2: Higher than normal values in your salt. Not all salts are the same, some have high calcium, some have high magnesium. If your salt is 500ppm in calcium but you only want 400ppm then you very well could keep up with only water changes, however you have to make sure to time your water changes such that you go below what you want so that those values don't creep up towards the value of the salt mix. Plus you'd most likely need to dose the other stuff as well.
3: Make your water change water higher to compensate. This absolutely would work, if you got the math down no problem. One issue though, this is basically the same as dosing your tank.
Conclusions
Even if you don't understand math, hopefully you can understand pretty pictures. The end result is you are not going to be able to get away with only water changes on your aquarium. You do need to dose to keep those levels where you want, period. You want a nice line that would straddling the 100% line on the graph, not the 70% line not the 50% line and certainly not the 5% line, even though that case most likely won't happen. Whether you have stony corals in the tank or all soft corals, there most likely is something in your tank that will need calcium and/or carbonates!
This however does not mean water changes are bad! In fact you could flip the meaning of the graph around to mean 100% clean water, and you can see how water changes are in fact a good thing!
Also this does not mean that you need to go out and get a calcium reactor, dosing pumps, or any other fancy stuff. If you have a low consumption then you simply could bring it back up every month with simple off the shelf products. However you really need to test to see how much you need to add rather than just blindly adding X cap fulls per gallon like many of the off the shelf products recommend. Also you should know how often you can test, since some elements register faster than others, i.e alkalinity changes are more easily registered than calcium ones (again don't know what, I'm not a chemist). So regardless of how your tank is setup, make sure to keep your inhabitants happy and make sure they have the elements they need in the water to grow happy and beautiful (or ugly if you dig that sort of thing!)
Bottom line is that will never work!* (* with some exceptions)
Abstract
If you get one of these "good" salt mixes that have the "proper" levels of this and that, and those are what you want, you will never have those levels in your tank with out dosing. If you have creatures that consume those elements, then you'll have less than what you started with, if you change the water then you're mixing a solution of less than ideal with ideal, the end result will be somewhere in between those two values since you're diluting your ideal concentration with something less potent and the result is always going to be less than ideal (just varying levels of how much less).
The test
So what I did was use excel to compute how much of a percentage of "it" you'll have left with various levels of water changes. The way it was set up was as a percentage of what we want, not some ppm or something, so it doesn't matter how large or small your system it works the same.
Assumption 1: Constant consumption of elements. Now I realize this isn't true, if there's 200ppm calcium vs 400ppm I'm guessing the uptake might be a little slower since the water isn't as concentrated. I don't pretend to know exactly how the creatures in our tank use them, my guess is that they'd simply stop consuming all together. I'm not trying to show you exactly how much you'll be left with, the purpose is to show you that you will never get back those initial conditions on day 1. The rate at which you lose will differ but again that's not really important, so take the actual percentages that are left with a grain of salt (or a whole bucket worth for later days!
)Assumption 2: These are elements/values work in a linear fashion, so 400ppm/8dkH they all work in that same fashion in that to have half as much of the value, you take half as much of what that value is talking about. Things like pH don't work at all like that, the what needs to happen to go from a pH of 8 to 9 is much different than from a pH of 7 to 8. Even though the consumption doesn't work in a linear fashion, because the elements themselves do when they get doubled/halved/etc the idea still is valid
Assumption 3: I set my consumption rate to about 5% per day of the element, whether or not this is a true reflection of consumption again is irrelevant, if your consumption rate is slower then it will simply take longer to reach a lower value. 5% however does reflect a calcium level of 400ppm and losing 20ppm per day, so it's not totally off. Again with #1 I'm sure this rate isn't constant, but that doesn't matter.
What I did was take 6 separate cases. First case was the worst case, and no water changes at all, then a series of weekly water changes at 10, 25 & 50% respectively, a 50% monthly and a 10% daily which is used to reflect a "continuous" water change that some people are using (I did 10% a day just to show what will happen, the rate at which you do a continuous water change will affect how high or low it will get). I did this over a period of 60 days since that'd be equal to 2 monthly changes.
The results - Analysis
No changes: No big surprise here, you don't change the water your values will drop as elements are used up. Remember more than just corals consume calcium too! Most likely what would happen in reality is any coraline algae you have will simply stop growing, maybe even dying back and new stuff won't grow back in it's place. Snails and other inverts that use it for an exoskeleton will most likely have issues as well as it can't make a new skin properly. The graph will eventually level out, however these will be at levels too low to be useful in a tank, anyway you slice it this is not the way to go.
Weekly changes: Again no big surprise it has a dip that's similar to the first 7 days of the no change, then there's an infusion of new water which boosts it up slightly, but you'll notice never back to 100%, even with the 50% weekly changes (which is a lot of water to be changing) after a few weeks it never gets more than 80% of the normal. As time goes on the rate at which it is used slows down, but regardless of how slow it goes it still is going to be less than the previous week, they do eventually get asymptotic however and bounce around an average, that average even at 50% is roughly 70% of what you started with.
Monthly changes: This in behaves just like a weekly change except the infusion has a much more profound effect in how much it will change. While the monthly mark brings the values higher than that of some weekly changes, you have to realize the average is still below the weekly.
Daily change: This was to reflect the continuous change, due to my resolution being 1 day, it does a fairly good job of it and as you can see has an asymptotic limit that occurs quite quickly as such you don't see the up and down infusions of the weekly & monthly changes. When reality pokes its ugly head in the math and you don't get a linear usage of elements, i.e. calcium consumption slows down, you would see those spikes with a daily usage, however the end result is still the change, if you put in X concentration in to a mixture of X-Y concentration your end result is less than X. That being said this one would probably be the most preferable if you were to use only water changes to keep your elements up, it won't keep them up where you want them, but they'll stay fairly high regardless. Just note that's a lot of water, 10% daily over 60 days will be 6x as much water used to change as the volume of the tank! (That doesn't even taken into account using an RO unit and the waste water too).
The Graph
The Exceptions
Early on I said there were some exceptions to the rule here they are.
1: 100% water change. Yup if you pull all the water out, and put in fresh water with every water change you will in fact have 100% of all the values you start with... at least until the creatures start sucking them down! However this is usually not recommended to do except in the most extreme of catastrophic circumstances with a tank. Also note, this only works if you first pull all the water out, simply putting water in while you pull out dilutes your mixture.
2: Higher than normal values in your salt. Not all salts are the same, some have high calcium, some have high magnesium. If your salt is 500ppm in calcium but you only want 400ppm then you very well could keep up with only water changes, however you have to make sure to time your water changes such that you go below what you want so that those values don't creep up towards the value of the salt mix. Plus you'd most likely need to dose the other stuff as well.
3: Make your water change water higher to compensate. This absolutely would work, if you got the math down no problem. One issue though, this is basically the same as dosing your tank
.Conclusions
Even if you don't understand math, hopefully you can understand pretty pictures
. The end result is you are not going to be able to get away with only water changes on your aquarium. You do need to dose to keep those levels where you want, period. You want a nice line that would straddling the 100% line on the graph, not the 70% line not the 50% line and certainly not the 5% line, even though that case most likely won't happen. Whether you have stony corals in the tank or all soft corals, there most likely is something in your tank that will need calcium and/or carbonates!This however does not mean water changes are bad! In fact you could flip the meaning of the graph around to mean 100% clean water, and you can see how water changes are in fact a good thing!
Also this does not mean that you need to go out and get a calcium reactor, dosing pumps, or any other fancy stuff. If you have a low consumption then you simply could bring it back up every month with simple off the shelf products. However you really need to test to see how much you need to add rather than just blindly adding X cap fulls per gallon like many of the off the shelf products recommend. Also you should know how often you can test, since some elements register faster than others, i.e alkalinity changes are more easily registered than calcium ones (again don't know what, I'm not a chemist
). So regardless of how your tank is setup, make sure to keep your inhabitants happy and make sure they have the elements they need in the water to grow happy and beautiful (or ugly if you dig that sort of thing!)
