What’s an RDP Refugia/Sump?

Recent posts have prompted me to explain this concept. I will try to be brief. If you want to read more about the benefits of an RDP Refugia, go to the article titled RDP pH Control and Filtration in the Feb. 2002 issue of the Sea Star Online (blue button to the left).

RDP is the abbreviation for Reverse Daylight Photosynthesis. This term was coined by John Walch, builder of SeaQuest, the first and still functioning Marine Ornamental Fish Hatchery. John spoke to our club about his RDP system just prior to publishing an article in FAMA(Freshwater And Marine Aquarium) magazine in 1997.

The RDP Refugia is a sump that is lighted at night while the main aquarium is dark. There are four benefits of the RDP Refugia. 1) algae scrubbing/filtration, 2) nightime pH control 3)food production and 4) inhibiting algal growth in the main tank.

The system requires water to be drawn from, and then pumped back up to the main aquarium. It also requires plant growth lighting to be set on a simple timer to come on before the main lights go off at night and to go off after the main lights come on in the morning. Most locations where the RDP is located, may require some kind of opaque covering to keep the light from illuminating the room at night. A simple length of dark fabric can effectively hang over aquarium stand openings, preventing light from shining out.

The RDP Refugia in the system pictured below is simply a 40 gal. acrylic box. It effectively grows Racemosa/Grape Caulerpa under 2 - 30 Watt NO white/plant growth fluorescent tubes. A small, reverse flow, air driven skimmer runs 24/7. The water is moved up to the main 180 gal. aquarium by 2 Rio 4500 submerged pumps. This system and it's equipment has been running uninterrupted (except for power outtages) for three years.


(The two gallons of B-Ionic sit full and unused!)
Water changes are easy by turning the PVC ball valve at the left of the pic below and letting one of the pumps push the water through a hose in the wall out to a bucket in the garage.


Top off is by Kold-Steril water connected through a sprinkler valve and toilet bowl float! Notice the black fabric covering the back opening.


Three years and still running! One of the two Rio's on the left and one of two water inlets on the right, in this pic:


The "inadequate" skimmer sits in the sump.


Soft Coral grows wild. Cutting/Propagation is performed quarterly.


An RDP is supposed to grow algae like crazy so that it doesn't grow in the main tank and algae growth filters by removing nutrients from the water and from the system as it is harvested. Pictured is the right corner, pump and inlet.


Bubble Coral recovering nicely from near death at the LFS.


For those astute hobbyists that noticed the Valonia in a pic above, this is the recent addition that has almost eliminated what was a fairly robust growth of the pesky Sailors Eye Algae.


Edited by Mark Peterson

If I may be so bold as to add to this post, I would like to post a picture of another RDP/Sump.  The picture below is my 10 gallon RDP/sump.  The RDP is on the left and the sump is techinically the entire 10 gallon tank, however, I usually just refer to the right side as my sump.  I have a Home Depot 6500 Kelvin screw in UV bulb that is 12 watts.  This RDP is 4 gallons so I feel that is sufficient.  If I had moved the cords out of the way and cleaned the diatoms off of the glass, it would look a little better.  I feel this way is better because the algae does not get to the pump to potentially cause it to clog, plus you can run your skimmer away from any pods that may get sucked up into it.  Even though this sump design may look simple, there have been many attemps at creating a design that works efficiently.  I urge any DYI'ers to consult a LFS that manufactures these and possibly purchase one.  I have seen many people look at this design and say, "I can make one of those," only to come back into the LFS and ask to buy one.

I like mine, but I prefer to call it a "mangrove greenhouse".

Ok - not to start a hotly debated topic... but I would like to know if running a skimmer after the refugium takes out any good things the refugium puts in... I really like the design of marcus' sump/refugium setup, and would like to implement it - but I am wondering about the post refugium skimmer .

coreyk wrote: Mark Peterson wrote: Most locations where the RDP is located, may require some kind of opaque covering to keep the light from illuminating the room at night. A simple length of dark fabric can effectively hang over aquarium stand openings, preventing light from shining out. i was wondering what are the adverse effects of not having some sort of opaque covering and letting the light shine out. ?

does anyone have an opinion about this?

COOL PCPB! That is the one sump to rule all sumps!

coreyk wrote: coreyk wrote: Mark Peterson wrote: Most locations where the RDP is located, may require some kind of opaque covering to keep the light from illuminating the room at night. A simple length of dark fabric can effectively hang over aquarium stand openings, preventing light from shining out. i was wondering what are the adverse effects of not having some sort of opaque covering and letting the light shine out. ? does anyone have an opinion about this?

As you can see from my sump/RDP, I don't think its a big deal.  The moon puts off light too.

As for skimming after the RDP, some of the other designs have the total amount of drain water going through the RDP.  I do not.  The intake for my skimmer is out of the way of my main pump.  That way, only a small amount of RDP water gets into the skimmer.  I run my skimmer all the time, but I don't feel that all the water that goes into my drain goes through it.  As I understand skimmers, they remove any proteinaceous (hence Protein skimmer) matter that may break down into ammonia.  That matter, if left in the tank, could possibly break down and feed algae that could grow in your main tank.

So Jon if I get what you are saying - it might not be a bad idea to run the skimmer post refugium?

No, I think it's a best practice to place the skimmer before the refugia if you are using a refugia as a place to grow little critters for the tank.  If your refugia is more of an algae scrubber (ie used as a filter) then it doesn't really make any difference which is first, imo.  The principle of mine is similar to Marcus's, although the skimmer technically is after the refugia, most of the water exiting the refugia preferentially goes to the return pump.

Mark, my monitor is a crappy 15" from costco (since my 19" blew up) but I do have a pretty serious video card so my settings are at 1024x768.

Coreyk, I know you weren't ignoring me.  Glad to be of assistance!

Jon, what's the hang-on filter you have?  Is it some Alumina media or something?