Overskimming. Myth or Reality

[biotoxin]

Ok so ive heard a lot of people crying about overskimming a tank and personally i think its bs, but id like to get some opinions here. What is the possibility of overskimming a tank?

 

[Biggen]

I have never heard of it, but then again, I haven't been around long enough to really now. Although, it sounds a lot like the "overfiltering myth" to me.

 

[jackdp]

IMO if you have corals and you don't mind having to supplement trace elements, iodine, strontium etc and keep feeding invert food all the time then skim to your heart's content. True, skimmers remove organic wastes that can breakdown into toxic molecules ie. ammonia and nitrite. IMO skimmers will inevitably remove organisms and compounds from the water column that can be beneficial to cnidarians and other filter feeders. Clams for instance need a source of nitrogen to feed and grow. For the record I run a skimmer 24/7, but I'm not happy about it

 

[reefrunner69]

I personally believe I add enough junk to my tank that I keep a source of nitrogen all the time. FWIW, I believe it is possible, for example if you were to put a ETSS downdraft skimmer rated for 250g tank on my 20g tank, it would strip the water pretty quick, this would lead to a loss of corals, which need a source of nitrogen. If you buy an appropriately sized skimmer, on that process at least 1 and upto 5 or 10 tank volumes per hour, then I do not believe you will overskim your water.

 

[jks1]

i agree with reefrunner, as long is the skimming capacity of the skimmer is comparable to the size of the tank- skim on!

 

[biotoxin]

here's an interesting reply i got from another board:

I've heard all the tales of how a skimmer will "strip your tank clean" of trace elements. The process of water fractionation relies on the hydrophobicity of molecules and their attraction to the water/air interface (i.e. that's why a surface skimmer really helps!!). Water soluble elements should be unaffected by this process....unless the added oxygen to the system helps to "oxidize" unstable trace elements (say Iodine/iodide which is an fairly reactive molecule and therefore unstable)?

As far as I know, skimming removes organic molecules (lipids, proteins, and other biological waste before it breaks down)...... how is that bad?

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[jackdp]

So in selecting a skimmer size DOES matter 8O Thats why I don't mind having a "crappy" prizm on my 20 gallon. It skims enough for my tastes, if I added one of these $500 super skimmers like some suggest, I might skim my fish right out of the tank. I think it is important to qualify overskimming by saying the skimmer must be appropriately matched to the tank and livestock load of the tank. A skimmer and pump meant for a 200 gallon tank can/will strip much needed nutrients from a 20 gallon tank.

 

[loganj]

And you have to admit...a 6' tall skimmer with a V-8 Chevy powered pump would look silly sitting beside your 20g.

 

[reefrunner69]

 

[wseaton]

And you have to admit...a 6' tall skimmer with a V-8 Chevy powered pump would look silly sitting beside your 20g

And still be less efficient than a cheapy Prizm or Seaclone if the intake and outflow of the 'Chevy' powered skimmer were close to each other as most hang on style skimmers are. Basically they just sit there and churn the same water.

I'm extremely skeptical of tank size ratings for skimmers since a much smaller skimmer that placed in a sump will do a drastically better job than a much larger skimmer that's hung on the back of a tank. The most important thing with a skimmer is that it gets full tank turn over by it's intake/outflow configuration and not just how much water goes through it.

 

[biotoxin]

technically all skimmers just sit there and churn the same water Is there a reason why sump based skimmers do better? Is it because of the water flow?

 

[loganj]

I think the point that wseaton is making is that, with most HOB skimmers, the pump intake is so close to the skimmer output that a percentage of already skimmed water is being pulled back through the skimmer. I think this depends a lot on the currents in the tank though. Also, a surface skimmer makes an unbelievable difference in the efficiency of any HOB skimmer. You can also locate the pump/powerhead away from the skimmer. The reason a sump skimmer does a better job is that it receives a fresh supply of water that has been skimmed from the surface of the tank. It also, in most cases, dumps the skimmed water into an area of the sump where it will not be recirculated through the skimmer.
On the subject of skimmer efficiency, the dwell time of the bubbles is what makes the skimmer efficient. The V-8 Chevy powered skimmer was a joke of course. If you are forcing water through the skimmer at such a high rate that the DOC coated water molecules are not contacting the rising bubbles, then your efficiency will drop. What you want is a stream of microbubbles rising slowly through a downward flow of water. This lets the water molecules collide with the bubbles and shed the attached DOC's. This is why counter-current skimmers work so well...bubbles always going up....water always going down. Co-current skimmers have the water and bubbles going in the same direction thus reducing the contact time considerably. Verturi skimmers are like super charged co-current skimmers in the way they function. Most of the good ones have a double or triple pass system that gives you a degree of counter-current effect as well as the initial co-current effect. I hope this makes at least some sense. If not, I can suggest reading (I think...can't find the darn thing right now) "Aquatic Systems Engineering" by P. R. Escobal. After you read it, you'll really be confused

 

[astride13]

I do not use a skimmer on my tank. My Ammonia and Nitrites test at 0ppm and my Nitrates always test below 5ppm without water changes. I use strictly biological filtration (dsb+fuge).

Corals such as goniopora live in high nutrient waters that are rarely clear. Also, some species of Xenia are commonly found where sewer is released into the ocean. On the other hand, corals that get most of their nutrients from light prefer extremely clear water. It is a matter of making your "environment" the same as your inhabitants are use to. (and I don't mean to use sewer as topoff )

 

[wseaton]

the pump intake is so close to the skimmer output that a percentage of already skimmed water is being pulled back through the skimmer

Yes....exactly. The in-sump skimmer would take advantage of the fact that *most* sump configurations are designed to have efficient out-flow/inflow without too much over-lap. So, the problem solves itself.

This gets us into the side issue of waterflow in a tank and one of my peeve's that I think a lot of reefers have too many powerheads (horizontal circulation) vs vertical water circulation. I tend to use more back filters for water movement simply because the large models move a huge volume of water from the surface to the bottom of the tank with low velocity.