lduncan

Now your getting sucked in by the complexities of sand storage and release. I don't want to go there. It's going to be too hard and time consuming for me to explain this to some people, and to be honest, I really don't have the necessary patience to do it. Just because you think your story is true wasp, it doesn't mean it actually is. But if believing it makes you feel better, who am I to argue. Oh, and I would still be interested in this anecdotal evidence you were talking about. Where is it?

It doesn't change the end result. Think about it. This is another one of those complications which come from the sand bed model. It's hard for people to get to grips with. Which again is why I don't like it. Perhaps if you look at it from the other perspective it might be more clear, the point of view of what these export methods "see". And wasp, where is this anecdotal evidence you talk of? Layton

The equilibrium point is the equilibrium point, changing the water volume doesn't change it. How many people are familiar with chemical equilibrium, specifically Le Chatelier's Principle?

If you model the sand bed as a non cyclic thing, to keep it simple, yes it will keep using nutrients. But at a slower rate, which results in the build up of nutrients back to where they were to begin with, as the tank nutrient input rate is the same. So just like the skimmer, you get back to where you started.

For it to work the way wasp says it works, the sand bed must know the volume.

because the rates are related to concentrations.

Not to sound like a "pompous twit", but that's the truth. I've spent a lot of time trying to describe, and giving examples which people can understand. Conceptually some of this stuff is hard to get to grips with, if you haven't had exposure to these types systems from a control point of view. It's even harder if you're trying to do it all in your head. Your post is the very reason why I said I don't like the sand model... There are so many things which can complicate the issue for people. The fact that they cycle the way the do, time lags, saturation etc... But fundamentally, the sand bed can NOT determine what the volume of water in the system is. So there is no way it has the ability to alter the equilibrium between it and the concentration of waste in the water, based on the water volume. Layton

Well first, your sand model isn't accurate. I've explained that. Second, the reason I say I don't like the sand model, is that it is a lot more complicated that the skimmer model. I had enough trouble getting across the skimmer model to some people. The sand one will confuse the hell out of some people. It's not for the reasons YOU are imagining. You obviously think your story is correct. But it has a fundamental flaw that makes it nothing more than just that... a story. That flaw again, in case you missed it is this: You have assumed that the import of waste into the sand is constant, no matter what the concentrations are doing in the overlying water are doing when you add the extra volume. You have failed to account for the dynamics of how waste gets into, and exits the sand. The rate of input of waste into the sand is going to be dependant on the concentration of waste in the overlying water column (among a slew of other things, but NOT water volume). Which means your story, as nice as it may be, is just a story. Which means that you still haven't shown any "other factor" which will result in the concentration of nutrients being lowered in the steady state from simply increasing water volume. There are very fundamental logical and physical reasons why adding water volume can not alter the steady state concentrations of nutrients in the water. If you take all the various removal methods operating in the tank, and look at how they get their waste in order to remove it. They have a certain "knowledge" of the system, that knowledge comes directly from their interactions with it. The have knowledge of the concentrations in the system. They do NOT have knowledge of the volumes in the system. Therefore simply changing the volume of the system, does not change those steady state concentrations. Layton

Hmm, I could actually see you writing fictional stories for children. ;-) Back on topic, with Joe's tank you can eliminate the skimmer because "it doesn't matter" But wait a minute, when I eliminated the sand thing because i said it didn't change anything, you got all upset saying it changes everything? Seems a little inconsistent there. Anyway, even though the logic is wrong, i'll go with it for the sake of it. Let's look at little Joey's model more closely. What Joe has failed to understand in his model, is the dynamics of how waste gets into, and exits the sand. The rate of input of waste into the sand is going to be dependant on the concentration of waste in the overlying water column (among a slew of other things, but NOT water volume). And is not actually constant throughout the change in water volume. Which, when you go through the maths, results in his argument being wrong, and actually, the concentration of waste still ends up where it started off, just like I was saying. Also i don't particularly like this sand example, it's much harder to conceptualise. As you have storage pools of nutrients which introduces time lags which can quickly confuse people etc. Some people had enough trouble getting to grips with the skimmer workings. Layton

So you're saying that in this case the same amount of waste is being removed, And the waste which may be leaching back into the tank, is still leaching at the same rate? Right? And you're saying, even though the waste leach rate is the same, because it is going into more water volume, then the ultimate effect is that the nutrient concentrations in the water is going to be less than what you started with, after adding extra water volume? Am I understanding what you've said correctly?

Now I see why you said that, I was referring to the specifics of vacuuming sand before throughout. That's why I asked for clarification, there's no need for you to get presumptuous. Well then for starters you can ignore those processes which are not affected by water volume, as they don't affect the steady state outcome. Focus on the "other factors" which may be affected by water volume. Then you have to look at how they are effected to see if they alter the steady state. Are there any factors that you think are dependent on water volume and will change the end result? What are they? Your explanation above has clarified this. So now we can agree that vacuuming sand doesn't effect the final steady state result. And my claim that increasing water volume doesn't change nutrient concentrations in the steady state still stands. Layton

This one: I take that to mean that water volume has absolutely no effect on removal of waste from sand, which directly implies that the removal of waste before and after adding more water volume is the same. Am I wrong? Then in the next breath you say that depending on the processes involved, it may, or it may not, be the same. Which is it? Well you said that depending on the processes involved, the mass of waste removed may, or may not be the same before and after extra water volume is added. I'm asking in the case where it is not the same, what has caused it to change? Layton

When you say that you are implying that if you increase the water volume, the export of waste from the sand is the same, are you not? But I thought few sentences ago you said that extra water volume doesn't affect it? Can you clarify this a bit more? Anyway taking what you said here, in the circumstances where you think it's not the same, what has caused it to change? Layton

Oh, you mean that thread where you berated me for trying to find out something?: http://www.fnzas.org.nz/fishroom/5-vt13 ... l?start=62 It wasn't arguing I was attempting to get a full understanding of something. Layton

I don't have time to step you through it at the moment. As for me copping out with non-specifics, How about you get specific with these "other factors"? You've said that they effect the outcome, so show me HOW they do that. Maybe you could tell me how you think the removal of waste from sand is affected by extra water volume? Are you saying that the mass of waste removed is the same before and after extra water volume is added? Are you saying that the mass of waste removed increases proportionally to the extra water volume added? Are you saying that the mass of waste removed reduces inversely to the extra water volume added? Or are you saying something entirely different? Layton

I'm saying think about it more. Think about the processes going on, does a change in water volume effect them. And ultimately do these change the final outcome. IE does it result in lower concentrations of nutrients. Layton

If I had more time, i'd persist with explaining, might do later in the week. But for now I don't have the time to burn. But in the mean time keep thinking about it. Add a constant output to the model and see what happens. You might get a surprise. Layton

No, when i'm done trying to explain things to people who don't make sense. Layton

Think about it a bit more wasp...

Oh God, i'm not going any further than this. I don't know how much longer I can go on with explaining this. Wasp, think rates of input/removal and what changes them, then think concentrations etc.

Because adding water volume doesn't change anything to do with vacuuming sand!!! The two are totally independent. Hence my saying earlier that you are bringing in factors totally unrelated to adding water volume, and trying to attribute their effects to added water volume. It's just plain irrelevant. Layton

your not listening, i've said that those variables have been considered in the creation of the model, and do exist in one form or another, just not in an explicitly controllable form. That's the part which you keep missing. so how does adding water change the processing of nitrate in live rock? how does adding water change the vacuuming of sand?

Umm, all those variables are actually considered in my model. Just because they don't appear as independent variables, doesn't mean they are not factored into it. The point i've been trying to make, which has gone over your head is that just because these variables exist, doesn't mean that you have to explicitly separate them in the model. It's quite acceptable to combine the variables into a scaling factor (whether linear not not), and focus on the variables of real interest, the ones that define the trends and endpoints. That model in that link shows exactly the same trends and endpoints as the one I proposed earlier. Just take a step back, then you might be able to see the forest from the trees. You're getting bogged down in detail, trying to include every possible variable, and making it controllable controllable. It's totally unnecessary. I'm not saying ignore them, but all you need to do is consider them, and there range of effect, and then bundle all the uninteresting ones into scaling factors, it makes it a lot easier to see what's really going on in steady state, ignoring the specifics of the transients. The whole idea of modelling is to make it as simple as possible to get the required information out of it. The more detailed the model the more specific information you can get out of it. For the purposes here, the model I came up with is good enough, and is totally consistent with the model on the site linked. Layton

Maybe you failed to understand it?

Maybe it was just over your head?