Is it possible to overskim a reeftank?


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I did some research on the internet, and I can't get a really good, solid answer. What do you guys think..?


I don't think it's possible to overskim a tank. The more the better. I think people might start getting into trouble when they start adding things to a very productive skimmer, such as a UV sterilizer, Ozone, carbon/phosphate reactors, etc and it ends up making their tank too sterile and there isn't enough nutrients in the water column for their tank life to feed on.

I also don't think it would cause any damage quickly either. I think you would lose some color on corals, growth would slow, but I don't think things will start dying right away.

Just my opinion though.
The common answer from the "experts" I have run across is that overskimming is impossible as skimmers don't actually remove any required elements from an aquarium (this is a broad statement and does preclude underfeeding and certain nutrients from "bottom feeders"). I can say that a skimmer too small makes for a lot of skim cup changes and a system that is not as healthy. One too large can be a pain to adjust. You want to find a skimmer that is sized appropriately to your system or one that is very easy to adjust. Also, a lot goes into the configuration of your system as to how effectively you are skimming. The biggest skimmer out there won't skim fully if you aren't getting the proper flow, pulling water into it from the correct area or you have dead flow areas. The focus should be on the design of the water flow, pulling water into the skimmer from the correct place and making sure you don't have a skimmer too small.

Disclaimer: The above is more reguritated info and little practical experience. Take it with a grain of salt.
I agree with Straegen. You shouldnt buy a skimmer to large because then its hard to adjust and it will pull out alot of clean water and not be very affective. If you get one too small you'll have to change it all the time. You should have one that is just the right size. I had the Coralife 65 gallon on a 20 gallon and I couldnt get it adjusted correctly. It pulled water out instead of nutrients. I now have it on a 75 gallon and it is rocking it.
Anthony Calfo has said on more than one occasion that he was able to get a full cup of skimmate a day using his becket skimmers. These were running on tanks w/ little to no fish population, often containing only ONE fish. If I recall correctly it was for his coral farms, so it would seem nearly impossible to over skim.
According to Bob Fenner, it is possible but not very likely. Skimming does pull out iodine and a few other trace substances. In my opinion, there are living filter feeders that munch on the same things that your skimmer scum is made up of. Granted they are not in as full of number in the home aquria, but leaving some things in the tank is not totally bad. Stick to a good brand, Aqua C, Turbo Flow, etc... and "around" the strength for your tank size.

MAttTVI, A coral farm would produce more skimmate then a good stocked fish tank anyday as far as I understand.
I personally don't think it's possible to overskim. At any point, would you ever pour skimmate back into your tank? No. So why would you consider getting a less-than-optimal skimmer and effectively leaving that gunk in the water?

However, I do think that massive skimming and a lot of biological filtration (read: live rock) can lead to nutrient starvation, in the form of low nitrates in the system. I've run across this myself and it's been">talked about before</a> on this board.

I also don't think you can have a skimmer too big... :)
I agree. It can't be to big. Now oversized will lead to a couple potential issues. I also think it depends on the type of tank. I think this is 100% true for SPS tanks but if it's a softy tank or a clam tank, etc then I think you are self defeating in a way meaning that for optimal growth and health you'll have to put right back in what you are removing!

1) They are more difficult to adjust if they are way oversized

2) Waste of energy and effort

3) The nutrient issue discussed above
Well to buck the trend above...I do not skim....tank has never been better..

I have one, but it's too loud so I never run....well maybe once a month for a couple of hours cause I paid $300 for it :)
It would depend somewhat on what you're stocked with as well. It is quite possible to over skim a clam tank if you're dripping phytoplankton since clams do much better with a steady low flow than targeted feedings.
I dont understand, how are large skimmers more difficult to adjust than small skimmers? Several people have said this, but I dont get it. ALL skimmers should be properly adjusted. Fish are not the only producers of waste- ever see that slimy or waxy film that comes off acros, leathers, or almost any coral? That is concentrated carbon organic. That equals proteinaceous pollutant- exactly what skimmers take out.

I personally would never run a marine tank without a skimmer, but I know people have with great success (see Todd's post). I think overskimming a tank is near an urban legend. Aside from Chris' dilemma, how many people can complain here that their tank is too clean? We, as aquarists, add so much stuff to the water body (ie- food, addditivesm, body oils, dust, etc.), I cant imagine that the water body would become starved.

Just my opinion, though.
I think it relates to the neck size and expected foam head. Higher rated skimmers expect a certain amount of crap (literally) to foam up to fill the neck. If your tank doesn't produce enough waste, it becomes trickier to get the neck to fill up and float a good dry skim into skimmer waste cup. If you get a skimmer way too large, your system may have to wet skim just to get a foam head big enough to push up past the neck. It isn't that you can't get a good skim out of a skimmer too large for the system, it just takes more adjustment (generally speaking). That said there are some skimmers that are huge and have excellent adjustment capability that could work on even the smallest systems.

Next consideration is the flow rating. Depending on design, some skimmer have a suggested flow rating and this can be quite high. If you have say a 50 gallon system and a skimmer with 500gph optimum flow rating you would have to turn your tank over 10 times an hour into the sump for optimal performance. It will work with less, but you would have just wasted money for a skimmer that isn't being used to its fullest.

Again I want to stress this is my understanding on how this works and not from practical experience. If I am wrong and I certainly could be, I would like to know where I misunderstood the process.
Here's why I disgaree. Foam, whether it is in the neck or skimmate cup, is not in the water column. That is the goal. The biochemical nature of skimmers wont allow the protein enriched foam back into the water column. Most skimmers I know of are set up by water level, not by foam level. The foam level will vary, but water level is controlled by input and output. The longer the water is in contact with the bubbles, the cleaner it will become.

Heres a simplistic way to look at it. A skimmer is to your tank, as soap is to your skin. You can't rub a bar of dry soap on your skin, and expect it to clean it. You need to make lather (ie- foam in the skimmer). These little bubbles attract the polar chain organics have molecularly. That is what takes iof the "dirt". Now, if you scrub you hands for 1 second and rinse, will the be as clean as if you scrub for 10, 20, 30, etc seconds?

The skimmate cup really does NEED to be filled. It usually does get filled as these organics are removed from the water, but if the skimmer is really tall, and the foam ias building and buidling inside the neck, you may not see skimmate in the cup. Is the skimmer working- absolutely.

A large skimmer will likely work much much better on a small tank, with no more adjustment than a smaller. If your goal is to get the skimmer to make something for the cup, then yes, you will needs to adjust the valves. But, your skimmer will be working, even if you dont see much skimmate.
Not the waste cup but the neck of the cup is part of the problem with skimmers that are way too large. Larger skimmers are designed to handle larger volumes of water and more waste. A certain amount of waste is needed for foam to push up over the neck and into the skim cup. The more you throttle the skimmer to push waste up the more "wet" the waste becomes and the sooner you have to empty your cup as you are getting water in it. If your tank doesn't produce enough waste the neck isn't filled up and ins't pushed into the cup. Generally speaking, the larger the neck of your skimmer the more waste you need for a consistent foam to overflow the neck. If the neck is too large, one day you will have skim going over and the next three your skimmer won't be pushing it over the neck.

I do want to point out that the larger skimming is removing the same DOS as a smaller one generally speaking, but it just won't fill the cup for removal without a lot more tweaking than a properly sized skimmer. Also, it takes a skimmer that is way and I mean way too big for the tanks bioload for this to be a serious problem. This was my main point. Not that it wouldn't work, but it would be harder to "tweak" and a waste of money as it isn't going to pull out any more DOS than a properly sized skimmer.

Contrast to a small skimmer that always pushes the waste up over easily. Two problems pop up from this. First, the cup fills up fast and second the skimmer isn't likely skimming all the waste from the water as it has reached capacity.

It also takes longer to "break-in" a skimmer that is consider too large for a system. I will have to re-read the science section on skimmers again and make sure I got it right, but I am pretty sure I understood that chapter. That said I have been wrong before and will be wrong again... probably real soon.
jmaneyapanda wrote: Foam, whether it is in the neck or skimmate cup, is not in the water column.

I think it all goes back to this point. It doesn't matter if the skimmer is oversized, as long as the organics aren't sticking around in the tank.

Interestingly enough, there's no quantifiable way to say either how much skimmer you need for your tank, how much a given skimmer can pull out, or anything else. You may be able to define overskimming, but it's hard to say when a skimmer is operating at it's max potential or if it could still be pulling more out. However, I can say with pretty good certainty that no skimmer will pull out 100% of the organics in the tank; the creatures will always be producing something. Thus, the larger the skimmer, the more likely you can make that water pristine.
mojo wrote: Interestingly enough, there's no quantifiable way to say either how much skimmer you need for your tank, how much a given skimmer can pull out, or anything else. You may be able to define overskimming, but it's hard to say when a skimmer is operating at it's max potential or if it could still be pulling more out. However, I can say with pretty good certainty that no skimmer will pull out 100% of the organics in the tank; the creatures will always be producing something. Thus, the larger the skimmer, the more likely you can make that water pristine.

I certainly won't disagree with the inability to measure effectiveness. Just from my limited experience I am finding skimmer setting more art than science. However, I have read several posts on skimmers that don't fill waste into the cup properly due to the neck size being overly large and not enought DOS. Who wants a three week old DOS floating at the top of their skimmer not entering the cup?
Yes, but my question is, why do you care where the waste is, as long as its not in the tank water? Whether or not the foam is overflowing to the waste cup has no bearing on how well the skimmer is working. Lets say I put a skimmer rated at 8 million billion gallons on a 10 gallon tank. Over time the foam will build and build, and build in the neck. But it probably wont reach the skimmate cup for A VERY LONG TIME! But that skimmer is still removing the wastes just as good as it would on a 8 million billion gallon tank. It is just accumulating less. What you see in the skimmate cup is not necessarily an assessement on the skimmers function or lack thereof, but instead of the accumulation that skimmer is producing. The set up, whether its on a 10 gallon or 8 million billion should be identical.
I just noticed a typo in my post #14. I meant to say the skimmate cup DOESN'T need to be filled.
I am not arguing it won't be pulling the same amount of waste as an appropriately size skimmer for the tank. They will both pull the same amount (again generally speaking).

It will likely take longer to break in the larger skimmer so you won't be skimming effeciently as quickly, it will cost you more money to operate and purchase and you will probably need to wet skim from time to time so you won't end up with some really foul smelling DOS sitting in the neck and not the cup.

I am only talking about skimmers that are WAY too big not a skimmer that is moderately larger than what is required.