Need some opinions on underflow vs overflow

[Monacle]

First of all, I would like to make sure that the term I have been given is the same meaning to me as it is with everyone else. My reef store owner highly promotes the use of a "plenum". A system in which you use eggcrate sitting on top of 1/2 inch pvc pipe laying on its side with mesh over the top to keep sand from getting down through it and then the sand bed over the top of this.
What is the benefits of having this dead water space beneath the sand bed?
My buddy and I came up with a solution to having things going down and out the overflows. We did the whole "plenum thing and then we have a drain going out the bottom of the tank. We put a 3/4 inch riser pipe coming up from the drain to the desired water heighth in the tank. Then we took a
2" piece of pvc and cut slits in it at the bottom all around the base about 2 inches long. Then we slid this pipe over the 3/4 inch pipe and cut the mesh around the pipe. So the water goes into the tank from the supply at the top, goes down through the sand, fills the 2" pipe and then the water goes out the riser and down the drain in the bottom of the tank. (note: must keep the top of the 2" open or if you cap it make sure you make an air hole or it will create a syphon.)
Are we breaking some cardinal rule by drawing water down through the sandbed?? If you want I can submit some pictures of how this is all going together or you can check out the thread in the breeding forum.. I will be adding pictures to the breeding project post almost every day now.


Thank you and any info in this area will be greatly appreciated.

[Diver_1298]

I don't have a strong opinion one way or the other right now. Here is an article that outlines the pros and cons without being biased imo.
http://www.3reef.com/sitemodules-Con...rticle-11.html

btw, I like your setup and hope it works out for you. We will all be better off if we can raise the fish ourselves and stop the destruction of the environment for a few dollars at the collection end of the business.

Jim

[Monacle]

Thanks Jim, I appreciate the link. Good info too. Anyone else got any opinions on this matter?

[Blade_Runner]

As far as I'm concerned a plenum is simply a variant of an under the gravel filter with a low flow rate. IMO, an under gravel filter becomes clogged after time and will have the same anaerobic zones. If I understand what you are doing, you have created something similar to this. I've heard that the better option is to create flow up through the bed so that it is less likely to trap detritus in the sand.

In a marine tank, the purpose is to provide a spot to anaerobically break down NO2 and NO3. The big question in my mind revolves around two issues.
1. Do you have a NO2 or NO3 problem?
2. Is a plenum any better than a DSB?

I have heard that as little as 1/2" down in a DSB you are already in a low O2 environment and that 3 to 4" may be overkill and just a place to accumulate detritus. Personally, I have gone with the "DSB" solution. In my 210 I really don't have room for a good fuge with a DSB in it so I have 300lbs of sand in the tank. I have low NO2 and NO3 readings. In my 300/90 combined system where I have a huge sump in the basement the DSB is in the sump in the same area as the macro algae. I have almost no detectable NO2 or NO3 ever and it is shallower with a higher flow rate over it than the 210's DSB.

Go figure...

I don't have a clue which is better of the DSB or plenum options. I think plenums are cool, but look like more work and maintenance. If it fails it sounds like a pain to clean, but so is a DSB (If it's in the show tank).

My personal solution is to give up in trying to figure out what is most efficient and just use a bigger hammer. Hence a 200+gal sump/fuge. If I were to do it over agian, it would be even bigger. Probably 300+ It's only a little plex and water.

[Covey]

You basically set up a under gravel filter I don't think it will work out in the long run. Like blade said if you don't provide a anaerobic area in you sandbed you won't get any denitifying bateria nad you should a slow build up of NO3.

[Blade_Runner]

I think his flow rate might be slow enough for O2 free areas.

Monacle, can you post a picture or design?

[Monacle]

Absolutely.... Here are some pics of the setup.
Here's a pic of the stand pipe. It's a 3/4 inch pipe drilled through the bottom of the tank.


Here is a pic of the oversleeve with the slits in the bottom. this will draw the water down through the sand and up the sleeve and then down the standpipe: Hence, larvae and snails and crabs and anemones won't go down the overflow.





Then the eggrate is cut around the over sleeve. Note: see that the eggcrate only covers a section of the bottom of the tank. There still should be a good anaerobic side on the opposite side of the tank right?


Then, the screen goes over the top of the eggcrate.


Then the sand goes in over the top



So.... what do ya think?

I am going to post a new thread on the sand filter.. I figure it's different enough to start a new thread.... so look for it under the sand forum. Thanks

[Covey]

Yep that is an elaborate UGF. What will the flow rate be?

[Covey]

Well I read thru your breeder thread the system might have some merit for the whole larve rearing thing but I thought most people raised larve in filter less tanks.

[Monacle]

The larvae will be collected out of the tank after hatching.... but I do not want the larvae to go down the overflow before they are collected. Keep in mind that the undergravel part only covers about half of the tank... the other half does not have a plenum in it so should have anaerobic area there. The flow rate is pretty low and should only flow water right by the standpipe so I figure there should be areas of anoxic zone in there somewhere also. I guess the thing to do is try it out and make modifications where necessary. Thanks for all the input guys. More ideas? I am learning so much here, it's fantastic.

GLTA