Does skimmer size/capacity/rating actually mean anything? Does it even matter?

Discussion in 'Equipment' started by Vincerama2, Jun 27, 2013.

  1. Vincerama2

    Vincerama2 Evil Overlord

    Hi guys, I set up a 100g tank with maybe 75 gallons in it to temporarily house some fish and rock while I set up my 180.

    The only skimmer that fits in the tank (it has fat perimeter bracing ... "eurobracing?") is a Coralife Super Skimmer 65. CookieJar was nice enough to give me an old skimmer that I can use in a sump, but it doesn't fit this tank with the euro bracing. I will be running a sump skimmer with the 180.

    Now the SS 65 was troublesome (overflowing, not skimming, etc) until I figured out that I need to stack stuff under it until the pump input should be 1/2" below the water surface (basically just setting the water level in the skimmer, since its neck does not extend to adjust). As far as I can works amazingly. It is pulling some dark crud out. Now ... it doesn't pull out a TON of skimmate, but isn't this really a function of water quality? The tank started with seasoned live rock (with some dead corals on it, admittedly) but also with maybe 60 gallons of freshly made salt water.

    But this had me thinking. The skimmer will process whatever water pumps though it. There is no other input other than the amount of crap in the water. I'm assuming the efficiency of every skimmer is more or less constant. So if you can skim out 10% of any organics in a given volume of water, that doesn't really change. The efficiency would be a function of skimming method.

    If the skimmer was larger would that make a difference?

    I can see if the PUMP flowed more water, then more water gets processed, but in theory, once your water is "clean" or mostly clean and the skimmer is running at its max only needs to skim out the daily input of fish poo/coral barf, right?

    So would having a massive skimmer really work better than a smaller skimmer? If you only need to skim out that day's poo, then isn't running the massive skimmer just a waste of electricity?

    And yes, it's true that the skimmer also works as a gas exchange booster, but let's talk about its skimming power.

    So, in theory, if I had a 1000 gallon tank, do I really need more than a skimmer "rated" for 100 gallons? Isn't skimmer size more a function of "fish/coral" load than actual water capacity of the tank? I acknowledge that larger tanks tend to have more pooing things in them than smaller tanks. But let's say the bioload is the same in a 100g tank vs a 1000g tank because you gave 10 fish.

    Discuss ....
  2. zeroinverse

    zeroinverse Guest

    That sounds right. It is load dependent. That is why skimmers always say "medium load, light load".

    But ultimately, if you are upgrading your tank and you eventually plan to add stuff, you will eventually need a bigger skimmer. ^_^
    Also, the other reason to have a large skimmer is not also the FOOD you feed your fish, but to handle any DIEOFF. If an anemone dies, or a sea cucumber, etc... those are things that a larger skimmer can help remove before it kills other things in your tank.

    My skimmer has helped me handle a large carpet anemone partial dieoff (on one vacation, it moved to pump - probably hungry), a large BTA dieoff (while I was on another vacation), and moving a toadstool (tissue sloughing/dieoff of torn sections)...

    For me the skimmer is less about "normal fish waste load", but the ability to skim all the "worst case scenario" situations.
  3. rygh

    rygh Supporting Member

    Fun question. Big tanks with small uncertain load is a tricky problem.

    Skimming is a function of throat size, body size, dwell time, pump, bubbles, and flow through the skimmer.
    Since we are comparing large/small, we can cancel out most of those differences.
    For a quality skimmer, body size, pump, are sized to flow. Bubbles etc are assumed good.
    Which really leaves only throat size and total flow through skimmer.

    The larger skimmers have a larger throat, so need a minimum level of crud to build up a head enough to function.
    So it is not optimal to put a 1000G skimmer on a 100G bio-load.
    On the other hand, a small skimmer does not have enough flow to get a decent percentage of the tank water through.
    So it is not optimal to put put a 100G skimmer on 1000G tank, regardless of load.

    So I don't think there really is a good solution.
    Running several small skimmers is a possible option.
    A skimmer with an adjustable throat is a crazy thought.
  4. Vincerama2

    Vincerama2 Evil Overlord

    So really, it's better to get a skimmer that is too small than too big, because before the big skimmer can operate ... the tank must be more polluted?

    One problem with small skimmers is small skimmate capacity. So you have to empty the cup more, but I guess drilling the cup for a sludge drain is feasible.

    Dwell time. Hmm, so putting the skimmer in the sump in a lower-flow area will allow the water to circulate into the skimmer, then back out then some of the previously skimmed water can come back in for a "second chance" of being skimmed.

    This might make a good case for a less gph return pump. The slower the water moves through the system, the more time the skimmer gets to skim the same water. On the other hand, skimming water that has already run though the skimmer a few times means that dirtied water is not being processed by the skimmer.


    BAYMAC Guest

    I've always advocated less turn over via sumps for just that reason.
  6. rygh

    rygh Supporting Member

    >> So really, it's better to get a skimmer that is too small than too big, because before the big skimmer can operate ... the tank must be more polluted?
    Better than "WAY" too big. Like the factor of 10X in the example.
    Going a bit bigger is different, and seems like a good thing.
    Especially since a lot of manufacturers exaggerate the skimming ability.
    Most people over-size a bit. I did.

    >> Dwell time. Hmm, so putting the skimmer in the sump in a lower-flow area will allow the water to circulate into the skimmer, then back out then some of the previously skimmed water can come back in for a "second chance" of being skimmed.
    First: I don't really know the physics of the whole air/water/Hydrophilic boundary process in detail, so a bit of a guess.
    I would say Yes/No.
    Dwell time = time the water spends circulating with the bubbles.
    So if there are lots of micro bubbles in the sump, yes, it probably helps to re-skim. And there usually are.
    The downside as you mention is that re-skimming already cleaner water is less efficient.
  7. sfsuphysics

    sfsuphysics Supporting Member

    These comments seem to be at odds with one another, you initially said skimming is a constant, but then you say skimming is a function of dirtiness.

    Think of it this way, lets say you have a skimmer on a 100g tank and it's an adequate skimmer doing it's job.
    You move to a 1000g tank but only keep the same fish, corals, rocks etc, only difference is more water volume. The density of pollutants in the water will drop by a factor of 10. Basically the water will be cleaner, so the old skimmer won't work as well (assuming your skimming is related to polluted argument holds true), but a larger skimmer rated for 1000g will basically work at the same rate (efficiency) and as in a similar fashion not as well due to pollutants being more diluted, but the skimmer will process more water, so it will pull more gunk out, even if it's doing so at a reduced rate.

    You'll need the largest skimmer simply because you need to process more water, it's not the pollutants you're processing it's the total water volume that you're processing since the pollutants will get spread out through all of it.
  8. rygh

    rygh Supporting Member

    Fun discussion.
    So I figured I better read up some more on the real physics of foam fractionation.
    Feel free to look up "Ward-Tordai equation".
    A decent link here:

    My opinion/interpretation of what I read and how it relates to discussion.

    1) Dwell time matters.
    Simply confirming, and makes sense. The more time there is a surface to act on, the more recovery.

    2) If a bubble is damaged you start over.
    Makes sense. If the bubble pops/reforms, the chaos of that drives most of the
    recovered proteins back into the water. The surface is tenuous.
    So to some extent, that will reduce the effectiveness of recirculating, since
    running through the pump again will have some negative effect on the bubbles.

    2) The concentration of pollution matters.
    I was glad to see this confirmed.
    Near the bubble surface, there is diffusion, then adsorption.
    So the dirtier the water, the more efficient it will skim.


    Given that, it rather seems that recirculating is not a good idea.
    A bit of a surprise, since it is supposedly helpful. Good old anecdotal evidence I guess.
    You want to optimize dwell time with a larger skimmer body, not recirculation pumps.
    And you want maximally dirty water, directly from display tank.
    Of course, I could be reading the literature wrong.
  9. rygh

    rygh Supporting Member

    The problem is, a larger skimmer also has a larger throat.
    If not for that, then yes, your argument would be totally correct.
    And with the larger skimmer and its larger chamber, it may also be less turbulent, and thus even more efficient.
    But if you lose the foam column, a skimmer does not work at all.
    And the large throat could do that if there is not enough to fill it.
    However, I really don't know at what point that will be an issue.
    Not an issue for 2X or 3X over-sized, but it sure seems like it would be at 10X.
  10. Vincerama2

    Vincerama2 Evil Overlord

    Actually I was just commenting on rygh's comment about the throat needing more poo to operate.
  11. sfsuphysics

    sfsuphysics Supporting Member


    So some points that come across is that there should be a "magic ratio" of skimmer pump volume to skimmer body volume.

    Dwell time is important, and while presumably dirtier water "floats" (is pushed up) in the skimmer and the water exiting is at the bottom I can see with a smaller skimmer what would happen is that all the water would reach some level of "saturation" so dirty water would in fact leave the skimmer as cleaner dirty water goes back in. Now this COULD be beneficial in that since all the water is dirty and the claim of #3 (the second 2) was that dirtier water makes skimmers work better, but if too much dirty water is exiting then any dwell time that was associated with it on bubbles is equally lost. And if dirtier water is exiting, then it really isn't doing anything.

    Also points to an interesting question on sump turn over. If you take extremes of either spectrum of a drop of water per minute movement all the way to the entire volume of water moving every nanosecond you can see what could happen. Slower turn over means the sump water will become progressively cleaner compared to that of the tank eventually have next to no "dirty" in it meaning the skimmer won't function at all. The fast turn over would mean the sump dirtiness is comparable to that of the tank, which means equally dirty water so the skimmer works better, the water would output would be slightly less dirty so it would still be effective, it's just that you'll actually be skimming the entire tank's water (extreme case 2) vs just the sumps water volume (extreme case 1). There also could be a happy medium between the extremes that is better in both cases, but I think an easier question is whether or not a skimmer in the sump is as effective as that same skimmer in the display tank.
  12. zeroinverse

    zeroinverse Guest

    Wooooooo.... I see an online graphing calculator to calculate "sweet spot" ratios based upon the following parameters... ^_^
    1) Sump turnover rate
    2) Skimmer "extraction" ability (throat diameter, air injection rate, skimmer mixing/turbulence capability)
    3) Desired "rate of waste reduction" (i.e. time to reduce X amount of waste down to a certain amount)
    4) Desired "density of skimmate" (watery vs concentrated)
    5) Preferred "time between cleaning skimmate holder"

    All this for a reefer to determine how to optimize their skimmer (or to size the right one)...

    Ok, I just gave myself a headache. I'll just do it by trial and error & approximation. ^_^

    Cheers & happy Friday!

  13. Vincerama2

    Vincerama2 Evil Overlord

    So ... how about running multiple skimmers? (A waste of electricty!)

    Then when there is low organic content, the smaller skimmer works more efficiently, as it has the thinner throat and more bubbles will make it to the top and be "skimmed" out. The larger skimmer will, of course, provide nice gas exchange until a fish dies or a coral spawns, or your nephew comes over and pours a full jar of fish food into your tank, then the large skimmer will kick in and save the day as it has the capacity to handle the high sudden bubble production, meanwhile the small skimmer may over flow (and possibly spill the already skimmed skimate back into the water.

    If overflow protection is needed, the smaller skimmer should have an overflow container that auto-shuts off (ping pong ball valve) and at worse case, skmmate overflows back to the sump.

    I would conclude that it's better to have a skimmer that is "too small" than one that is "too large" as the small one will still skim for you, and you can tell that if it overflows with DARK skimmate (versus overflows with mostly water) then you need to go a size up. If you start too large, you might not reach the skimming threshold at which you can actually skim ANYTHING and so you curse your tank to a higher level of dissolved organics at all times.
  14. sfsuphysics

    sfsuphysics Supporting Member

    Also note, that many skimmers (even among the same brand) will use the exact same pump, the Sicce PSK-2500 comes to mind. So higher capacity doesn't always translate to more electricity.

    However using two smaller ones almost always will use more electricity, well unless you're like Chicken (Chris) with that monster he had which I think had like 3 Eheims just as recirculating pumps... that would use a few more watts :D

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