Ever heard of a plenum? - Substrates - Aquatic Plant Central

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Old 05-27-2020, 03:30 PM   #1 (permalink)
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Default Ever heard of a plenum?

I just recently heard of it. It's a gap of space under the substrate. It's basically an under gravel filter but no uplift from an airline or pump.

The idea is to provide low O2. No O2 is when anoxic condition occurs. It would work for dirt substrate too if you layer a garden fabric so the dirt doesn't fall through.
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Old 05-27-2020, 04:40 PM   #2 (permalink)
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Those have come in and out of favor at least 5 times in the last 20 years for reef tanks. I would not want one in a planted tank as roots would go thru he plenum pretty quickly
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Old 05-27-2020, 04:45 PM   #3 (permalink)
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Default Re: Ever heard of a plenum?

Why did it fall out of favor in the reef world?
A layer of the garden fabric would stop roots.
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Old 05-27-2020, 07:23 PM   #4 (permalink)
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Default Re: Ever heard of a plenum?

How does the water in the plenum get more oxygen when what is there is used up? I haven't tried it, but I don't see what problem it solves.
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Old 05-27-2020, 07:32 PM   #5 (permalink)
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Everything does. 20 years ago turf algae scrubbers was the hot ticket. Them came more powerful skimmers, then bigger skimmers and nitrate reactors, then chemicals, the fuges.... guess what they are back on? Turf scrubbers only they call them " algae reactors" or " algae chambers" the only thing new is they are now vertical instead of horizontal and they use led lights not vho
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Old 05-27-2020, 07:39 PM   #6 (permalink)
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Default Re: Ever heard of a plenum?

From what I understand,O2 levels decrease as you go down the substrate but it doesn’t go down to zero with the plenum. In low O2 environment, different nitrifying (denitrifying) bacteria consume NO3. Since O2 is low, they take O3 from NO3. By product is N2 gas. They do need carbon for energy. Sounds a lot like a compost pile.

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Old 06-04-2020, 08:01 AM   #7 (permalink)
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Default Re: Ever heard of a plenum?

Quote:
Originally Posted by mistergreen View Post
I just recently heard of it. It's a gap of space under the substrate. It's basically an under gravel filter but no uplift from an airline or pump.

The idea is to provide low O2. No O2 is when anoxic condition occurs. It would work for dirt substrate too if you layer a garden fabric so the dirt doesn't fall through.

I have two tanks, a 65 and a 6 (Fluval Edge) with plenums under a deep substrate of a mix of Oil-Dri (baked clay absorbent) and Eco Complete (about 3:1). The 65 gallon has a 3" bed with a 1" plenum and the Edge has a 1.5" with a .75" plenum. These tanks are 2 years old, moderately stocked with fish, snails, shrimp and plants. Both have HOBs.



My thoughts on the process came from reading up on Jaubert, Novak and the video series from Jay's Aquarium which I highly recommend for a plain explanation of the anoxic denitrification process, which is the whole point. From a reef perspective (which I'm not), I also drew on the GARF group who advocated a more open plenum system meaning more water circulating. The freshwater systems I'm experimenting with only have access via migration through the substrate and separating mesh.



In retrospect I'm not sure a plenum is needed, maybe only the deep substrate but I'm a believer in anoxic conditions for denitrification. My nitrate levels run around 20-30 ppm which I find very acceptable. I don't do a lot of water changes, maybe 40% once a month or so, and I don't do a lot of gravel vacuuming as a lot of waste breaks down naturally and is absorbed into the substrate. I'm on a well and use that water from the faucet. It's by then passed through a Whirlpool household water softener and a GE whole house filter.
My plants and animals are thriving and healthy.
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Old 06-04-2020, 08:19 AM   #8 (permalink)
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Default Re: Ever heard of a plenum?

yeah, It sounds like a plenum isn't needed in a planted tank. The plants will handle any nutrients. I think it might be good for anoxic dead spaces in the substrate, especially under large rocks and driftwood.

I have a small goldfish pond with a gravel bog filter with plants. It gets water changes when it rains. Last time I tested the NO3, it was 0ppm.
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Old 07-01-2020, 12:26 PM   #9 (permalink)
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Default Re: Ever heard of a plenum?

I'm curious if a plenum could resolve anaerobic/anoxic situations.




About 2 inches of top soil and 2 inches of sand, about 1 inches of space beneath the substrate.
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Old 07-10-2020, 01:07 PM   #10 (permalink)
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Default Re: Ever heard of a plenum?

*9 day update*
There are bubbles in the substrate but doesn't smell of sulphur, probably CO2 or trapped air.
It has been 90F around here. O2 can't be too much in this container.
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