A debatable thread on water turnover

[Dale W.]

This thread is created in reference to another thread asking about acceptable turn over rates in a marine tank. It's purpose is for discussion and debate so please keep it civil :)



The question at hand: Water circulation and calculation

High volume vs. high pressure.



We of course have to keep in mind all of the variables here with the type of aquarium, size and inhabitants.

Water turnover can simply be calculated using the specs on the pumps that we use, or can they? IMO, I don't believe so.

First things first. Most pumps are over rated in their pumping capacity for sales purposes. There are some manufacturers that are pretty close but most are not. This is also rated on a brand new pump in optimal condition. Pumps over time will gradually lose pumping capacity due to electronics, corrosion, and bio build up in the housing and impellor.



Water turnover rates:

We must first assess the reason for water turnover and it's benefits. O2 exchange within the water column is probably one of the biggest benefits to a high turnover. Along with this is the ability to distribute the O2 rich water throughout the aquarium. Another benefit would be the ability to circulate food. This is a very important factor in the case of a full blown reef for distribution of food on a planktonic level. The third reason would be particulate matter or detritus in the aid of removal.



O2 exchange IMO is the first and foremost that should be thought of when water turnover is discussed. O2 exchange occurs at the surface. For this reason, on a side note, you can include your return pump from a sump set up in the calculation as long as you factor in head height, bends, etc. into the equation. There is a substantial amount of O2 generation created in an open sump set up as well.

O2 exchange rates, volume vs. pressure:

Again many variables to be considered here. IMO this is where a pressure pump comes into play. If you are using a pressure rated powerhead pointed towards the surface, you will get substantially more surface agitation and O2 exchange than a volume pump can provide of the same capacity. In larger tanks, the pressure pump will distribute the water much further due to its velocity. Pressure pumps will also supply areas with saturated water better than volume pumps of the same capacity.

Even though it may sound like I am promoting pressure pumps, they do have drawbacks as well. To much velocity towards certain inhabitants can of course be harmful or fatal so this has to be taken into consideration.



The optimal set up:

For me it is a combination of pressure and volume. Pressure pumps are utilized at or near the surface or pointed towards the surface to get maximum O2 exchange and saturation where volume pumps are used to keep the lower end of the tank "moving".



As far as a number for aquarium turnover in respect to O2 generation and distribution, IMO pressure pumps are more in relation to static numbers than volume pumps. In other words, on a 100g aquarium with pressure rated pumps rated at a total of 3000gph would have a turnover rate of 30X(optimally) where a volume, wide dispersed set up at 3000gph may only have a turnover rate of 1800gph in relation to water movement. I haven't done the math or taken O2 readings in a controlled environment so these are only speculations on numbers. I think you get the idea though as to where I am going with this. So, to achieve the same benefits as a pressure rated set up, you would have to just about double the pumping capacity of a volume pump set up.



Comments?

[schigara]

I don't know. It's all too much thinkin' and rithmatic. I let the polyps on my SPS tell me if there is enough flow in the tank.

[excuzzzeme]

I think in your well-written piece there are aspects not addressed. 30x an hour for a 100 gallon tank is over-kill, IMHO. I doubt that a t/o 0f 30p.h. vs 10p.h. would create a higher amount of O2 within the column due to surface size constraints. Air pumps and air intake pressure pumps in themselves do not create a higher O2 rate. Air injected into the water column allows for faster off-gassing of some of the DOC's through surface agitation thereby allowing for greater O2 absorption.

A t/o of 30p.h. might well be advisable if you have a high particulate concentrate. I think you have brought up some very valuable information that should be considered when doing a set-up and needs to be researched further. I would really like you to continue in this thread with what you have learned about it as I for one would like to learn more as I feel that the hobbyist is not being told the truth but is being sold a sales gimmick where it comes to filters and pumps.

[Dale W.]

Excuzzzeme,
The 30X for a 100g tank was a simple referance number and not to be taken as an actual occurance.

"Air pumps and air intake pressure pumps in themselves do not create a higher O2 rate."
Exactly, and this was something that I wanted to point out but did not want to extend the thread into to many realms.

"Air injected into the water column allows for faster off-gassing of some of the DOC's through surface agitation thereby allowing for greater O2 absorption."
Again, yes. The key words being "surface agitation" which is where the issue of pressure vs volume can come into play. You will actually gain a greater O2 absortion with a higher turnover and agitation of the surface. Example, if you have a 10 gallon tank with no surface agitation, you will simply have the surface in sq.in for exchange. If you add surface agitation producing 1/4" peaks and dips, you will have doubled the surface area. Thats not exact math and numbers as I am just illistrating a point. So, yes, you are limited to surface area but we can bend that area a bit.

If you take a peice of paper and lay it flat, you will simply have that area as it stands. If you take that paper and crumple it up, you will still have the same surface area but now you have compressed that paper into a much smaller area. Now you can take several more peices of crumpled up paper to fill in the same area as the flat peice of paper. Make sense?

Oh, and to further extend your point on air bubbles in the system. You are right. You wont get any O2 exchange as the O2 molecules are being compressed to a point where they can not be released until they break the surface.

[Fishieness]

I think in your well-written piece there are aspects not addressed. 30x an hour for a 100 gallon tank is over-kill, IMHO.

It all depends what you are keeping. FWIW: I have a 90X on my 10 gallon, had 86 on my old one, and my old 40 breeder had an 80. Yes yes yes yes, it wasnt "exactly" 86 times turnover rate, but it gives a good estimate. It all depends what corals, fish you are keeping. I would personally never have anything as low as a 30X turnover rate on any of my sps tanks.

[OgreMkV]

One thing that bears mentioning is the type of flow. Turbulent flow is better for the animals (corals and such) and also better for making sure there are no 'low flow zones'. Laminar flow (which is produced by most powerheads and pumps) isn't so useful and can cause damage to corals at lower turnover rates than one would think.

Turbulent flow would affect the surface chemistry (gas exchange) more than laminar flow. I've seen pump setups with a 50x turnover rate, yet the surface was effectively unmoving. The return and intake were well below the surface and totally laminar.

[Amphiprion]

Well, neither powerheads nor airstones are all that great at oxygenating the water, believe it or not. Neither are protein skimmers (especially when there are larger tanks involved). What powerheads can do is move that oxygenated water around in a more efficient manner and promote diffusion and, therefore, equilibration. That includes both with the atmosphere and the static microlayer that surrounds corals. The BEST thing is actually photosynthesis--by macroalgae, etc. When coupled with the lesser effects of protein skimmers and strong water movement, it maintains a decent equilibrium with plenty of oxygen (even supersaturation at times).

DOCs cannot generally be gassed-off, unless they are volatile organics to begin with--most of which aren't. I assume you mean CO2...?

That being said, if you look at the ocean in terms of flow, it is mainly sheer volume. There the velocity and linearity are not so much pronounced as the overall volume. That is why I prefer a heavy amount of diffused, turbulent, high volume flow. That isn't to say that some higher pressure, laminar flow doesn't have its merits. I find it can be quite useful in influencing flow direction and perfecting a flow regime. It can easily alter the flow patterns of a high volume, low velocity pump. Also, some velocity helps permeate that microlayer I mentioned earlier a bit more effectively.

I also agree that turnover often does greatly depend upon what types of corals are kept. I have, however, had good luck with very high turnover rates and corals that are typically considered "low flow," such as bubble corals, open brain corals, etc.

Edit: Dale, I may be misunderstanding what you said, but I am not quite sure what you mean by a 3000 gph high volume pump only having comparable 1800 gph turnover... Turnover is turnover. While it may not influence the water column as violently, is moves the same volume of water--just over a larger area.

[Dale W.]

Good points on all.
I did mention that there are outside veriables to be concidered on the subject of flow but I dont want to stray to far into those realms on this thread. The basis of this thread is to get opinions on flow rates and turnover rates in accordance to pump design.

Amphiprion:
This is what I meant.
Yes, water turnover is water turnover with the pump itself. But is it the same in the water column?
For experimental purposes I will go to the extremes here. we will use a full 55g bare tank, two pumps, and a zero gravity ball. This of course is hypathetical as I am not actually doing this but may when I have time. The two pumps will both pump 1000gph. One pump is a pressure pump and the other is a volume, wide dispersion pump. The pumps will be mounted at one end of the tank, one at a time, with the center of the outlet 4" under the surface. start with the volume pump, run it for 10 minutes to stabalize circulation, drop the ball in the water. In 10 minutes time, how many revolutions will that ball do from one end of the tank to the other? Repeat the same for the pressure pump. I believe that you will find the ball doing several more revolutions with the pressure pump. This all translates into water movement(turnover).

As far as the ocean goes. There are all types of conditions out there from turbulant to extremely calm. Each area has it's own species that thrive in the given areas. Most all of the corals also need photosynthesis and the Macroalgae within the corals has a symbiotic relationship with the coral. Without the macroalgae, the coral will die. This is not true for all corals such as with Tubastrea sp. which actually thrives in caves.

Remember that this is not to say that one pump system is any better than another. It is just to define the two. This could help a lot when giving advise to someone on a system.

Water turnover is water turnover, hmmmmmm IMO that is like saying "well, if the fish is gold, it has to be a goldfish." :)

Here is a question for you. an acceptable average turnover rate, and I use this term loosely, is 15x. DO you think your tank would do just as well using a volume pump at that rate vs a pressure pump at the same rate placed in the optimal position?

[Dale W.]

BTW,
I wanted to touch on a point that OgremkV made in relation to gas exchange. "Turbulent flow would affect the surface chemistry (gas exchange) more than laminar flow. I've seen pump setups with a 50x turnover rate, yet the surface was effectively unmoving".Very valid point and part of what I was getting at. CO2 is not really a factor but NO2 is which is the final stage in the nitrification denitrification cycle which is why DSB's are promoted for the creation of anarobic bacterias. NO2 is a gaseous form that is expelled through the surface.

Nice info guys, keep it coming.

[Amphiprion]

Good points on all.
I did mention that there are outside veriables to be concidered on the subject of flow but I dont want to stray to far into those realms on this thread. The basis of this thread is to get opinions on flow rates and turnover rates in accordance to pump design.

Amphiprion:
This is what I meant.
Yes, water turnover is water turnover with the pump itself. But is it the same in the water column?
For experimental purposes I will go to the extremes here. we will use a full 55g bare tank, two pumps, and a zero gravity ball. This of course is hypathetical as I am not actually doing this but may when I have time. The two pumps will both pump 1000gph. One pump is a pressure pump and the other is a volume, wide dispersion pump. The pumps will be mounted at one end of the tank, one at a time, with the center of the outlet 4" under the surface. start with the volume pump, run it for 10 minutes to stabalize circulation, drop the ball in the water. In 10 minutes time, how many revolutions will that ball do from one end of the tank to the other? Repeat the same for the pressure pump. I believe that you will find the ball doing several more revolutions with the pressure pump. This all translates into water movement(turnover).

As far as the ocean goes. There are all types of conditions out there from turbulant to extremely calm. Each area has it's own species that thrive in the given areas. Most all of the corals also need photosynthesis and the Macroalgae within the corals has a symbiotic relationship with the coral. Without the macroalgae, the coral will die. This is not true for all corals such as with Tubastrea sp. which actually thrives in caves.

Remember that this is not to say that one pump system is any better than another. It is just to define the two. This could help a lot when giving advise to someone on a system.

Water turnover is water turnover, hmmmmmm IMO that is like saying "well, if the fish is gold, it has to be a goldfish." :)

Here is a question for you. an acceptable average turnover rate, and I use this term loosely, is 15x. DO you think your tank would do just as well using a volume pump at that rate vs a pressure pump at the same rate placed in the optimal position?

Good points, but I think you are missing the whole advantage of high volume, low velocity pumps. The point is you can get MUCH higher flow volumes on a tank without damaging inhabitants. For example, my 75 gallon tank has nearly 10000gph running through it. Now, if I were to put a 10000gph Hayward pump on a closed loop on my 75, it would blow the flesh off of nearly everything (probably fish, as well, lol). What I mean by turnover is the volume of the actual movement. The displaced volumes will be the same for both pumps--one will just be shooting a concentrated, higher velocity stream. I suppose I should have more precisely said "volume is volume."

By comparing the two with the same volume, you aren't really proving anything. In fact, I can say in your experiment, that the higher pressure pump would move the ball faster and with more revolutions. Simply put, it is pushing the same water volume, but at a higher velocity. It will move faster, period.

On reefs, the VAST majority of water movement is sheer volume. This can be substantiated just by looking at the tides. Even then, look at wave volume and the way it looks flushing past corals. It is relatively low velocity (under normal circumstances, since that is what most shoot for) and high volume. Granted, there can be high velocity surges, but in those areas, there is less coral diversity and it tends to break them up, as well. You just aren't going to find much in those types of situations--mostly algae and a few hardy corals.

Now for the last portion. Assuming 15x is an optimal turnover, and I happened to be keeping corals, then a higher volume, lower velocity pump would likely be more optimal for above reasons. I can provide the corals with mass water movement without blowing the tissue off of their skeletons. Keep in mind, though I don't mean very low velocity. A little is necessary.