Plenum in my 125gal

Alright i want to put a small plenum in my 125 before i add my 2" sand bed. I’ll post pics of what i have so far and tell me if it will work. It’s eggcrate base with 1/2in pvc goin up into tank with 1200 MJs on the top for suction. From my lil experience with this I need all th advice i can get.

i have plenums in most of my tanks. they work good, as long as you keep the sand clean of detritus. if too much stuff collects in the sand bed, it clogs up the space between the gravel grains and turns it anerobic . and then you get problems. otherwise they are pretty much fool proof ways of denitrating. but you need about 4 inches of the 2 mm aragonite sand or a mix ofthat and crushed coral over the plenum to make it work .

Garf is where i got my first info 10 years ago.
and it looks like thats where you got your diagrams too. just remember, 4 inches of gravel over the plenum. it needs that depth to make an oxygen gradient. Garf uses it like an under gravel filter for the first couple months with powerheads on the lift tubes to draw water and bacteria thru the sand, and activate it. then they remove the powerheads, fill the lift tubes with sand and give it another month or two while the bacteria make the oxygen gradient in the sand and plenum. then it works. seeding the tank with bacteria booster, and or some live sand helps it along too. long term the trick is to leave as much of the sandbed uncovered by rock or stuff so that it can breath. and keep it sand cleaned a little on the top layer.

Thx for the info ken! Man this might be a lil more in depth that i thought…

So yea can’t find my camera so guess no pics till it shows up… Anyhow would I be better off using the crushed coral/sand from my old tank mixed with some new? Also i’m looking at this plenum as really an under gravel filter so idk which would be better… Would having the 4" bed with the plenum running it every few months be worth it? Running it more often or constantly wouldn’t allow the bacteria to grow right? Hmm i might just go with a 3" bed total with no plenum considering might be moving in the near future…

I think the general opinions on sand beds is either keep it under 2" or at least 4" anything in between apparently has the propensity to go bad (much easier anyway) and release sulfur in to the DT. This of course is reitteration of previously read/researched documents as I haven’t had a sandbed go bad on me yet and can not speak from experience.

you have to note the difference between an under gravel filter and a plenum. the UGF or a plenum with with a powerhead drawing water through it is nothing more than a bio ball nitrate factory. its aerobic bacteria working only, converting waste to nitrates.

a sealed plenum acts like a weak battery. as the bacteria use the oxygen in the sand bed and in the plenum space below the sand, you get a stratification from aerobi to nearly anerobic, anoxic bacteria moving down in the sand. the change in orp creates a charge diferential between the low O2 plenum and the high O2 tank water. this attracts the nitrates down to be consumed by anoxic(nearly now oxygen needing) bacteria and converts it to N2 gas to bubble out. there is constant migration of nutrients and some O2 up and down in the plenum sand bed. the plenum and sand thus never go sour and turn to hdrogen sulfide like a simple deep sand bed does in no time. a deep sand bed alon, on the bottom is a one way street mostly taking in nutrients faster than it digests it and expells it and goes thru the hydrogen sulfide cycle.

so i like plenums. less chance of a sulfide release, one less thing to worry about. and it denitrates.
I have a 10 year old plenu in a 75, one in my fuge, the past 3 years and one in my two frag tanks. zero nitrates.

Read this part carefully:

[quote=“kaptken, post:6, topic:1607”]
you have to note the difference between an under gravel filter and a plenum. the UGF or a plenum with with a powerhead drawing water through it is nothing more than a bio ball nitrate factory. its aerobic bacteria working only, converting waste to nitrates. [/quote]

To me it looks as if what you have diagrammed in the top is a Nitrate producing undergravel filter. It might help if you are planning on growing sea grass, but with corals and sea grass I think it would just be a negative.

So if i keep the “Plenum” sealed after initially priming it how long until i run the powerheads to cycle the water underneath? Or do you ever touch it again? Kinda confused on the whole process after set-up.

never run water thru it again. its a closed, passive system. just an open water space on the bottom of the tank covered by sand. as soon as you pump oxygenated tank water in it you destroy the gratient to anoxic bacteria which does the work. and you go back to a nitrate factory UGF. all you need to do is keep the dirt from building up and blocking the sand bed. a little gravel syphoning once a month or two on the top inch or so. do not disturb the deeper layer on the plenum, that will let a slug of oxygenated tank water infiltrate the plenum and then it has to cycle again.

im slowly taking apart my 10 year old 75 for an upgrade. the base rock i buried in the sand on the plenum 10 years ago is snowy white, fresh smelling just like the day i put it in. same with the sand bed. and you find nothing but clear water in the plenum. when i had deep southdown sandbeds on the bottom, no plenum they got very sour fast with supfides. even the frags i layed on the top would be black and smell strongly of sulfides in a few days on the bottom. because there is no movement through the sand. a much slower anerobic process. the plenum is much more efficient at converting nitrates to N2 gas, in my experience.

oh, one little detail many people forget and may cause it to fail. the plenum should remain in total darkness. its not for viewing. otherwise algae may grow from light shining in and mess it up. i taped black plastic gargage can bags to the bottom glass of my tank to keep light from the sump area from shining up into the plenum.

It is very possible to run a tank with out a plenum and a plenum improperly set up can cause more problems then it helps. Until you understand better what it is you are setting up I personally would advise you not to start instillation. Either read up on the subject more or use other methods of filtration. Personally I would recommend reading over Ken’s words again then find at least two articles on plenums from reputable sources and read them through in their entirety.(and I said articles not forum posts and reread them until you understand them) I have took care of many tanks none of which have had deep sand beds or plenums.

Thx for all the info. just tired when i read these posts lol. I decided not to go with the plenum this time cuz i might be breaking everything down in the fall for possible deployment. Unless I can find a caretaker for 4 months… nudge nudge Joe

I’m here for you man. Gimme a call

Hey. Where you goin? Im sure we, someone, can find a place for your livestock for the duration. just say the word. When? Ive been holding and growing a coral for a friend for more than a year. he just hasnt gotten set up yet to take it back. but he will get it back. Spread the stuff around if you have to. it will grow in the mean time and you get back a stronger piece. uhhh/… hopefully. we are all amateur reef nerds, so no guarantee. but it’s worth a try.