View Full Version : DSB for freshwater
Does the deep sand bed method used for saltwater work for fresh water?
Andy16
07-23-2003, 3:59 PM
It will probably colonize more bacteria but it will also be a place for wastes and food to decay. So i dotn think it would have the same effect.
Why would the food not waste and decay there in saltwater?
I have never kept sw so I am not really familiar with it. I was just reading about dsb.
Hebdizzle
07-23-2003, 4:04 PM
saltwater tanks have more invertebrates that stir up and help clean the DSBs. crabs, starfish, worms, seacucumbers etc
aaron
Also with sand packing so tight it is an opportunity for anerobic bacteria to breed and release gas that might destroy your FW tank. Wait a tick, I'll see if I can drag up that thread.
Andy16
07-23-2003, 4:19 PM
I was going to say the same think as matak but i forgot what hte bubbles were called. And in a SW aquarium, the creatures in the DSB eat the wastes. Also waht hebdizzle said.
Can't seem to find that thread about the disasterous gas percolating in one members tank. The gas given off really dropped the pH and killed the entire tank. Any clues?
Andy16
07-23-2003, 4:24 PM
I remember one thread about a lady that has kept fish for like 15 years. She was doing a water change and 3 little bubble came out of the sand and then 10 seconds later all of her fish were dead.
Hebdizzle
07-23-2003, 4:57 PM
http://www.aquariacentral.com/forums/showthread.php?s=&threadid=9863&highlight=anaerobic+gases
CHINABOY1021
07-23-2003, 6:15 PM
wow, its the aquatic plague.
Since I was in one corner of that thread, I'll chime in here as well. That thread was never resolved, as we really don't know what killed the fish. Ammonia, methane, H2S, lots of acids, and many other things of varying toxicity were no doubt available in the overfed tank (not being nasty, it was a fry tank, everybody overfeeds there - practically a requirement).
But it was the disturbance of the overly deep substrate that triggered it.
I have not done FW DSB, but have multiple FW plenums and they do not go anoxic. Anaerobic, yes, because they are denitrifying, but not anaoxic - I took one of them down early this year to check, no H2S, no iron precipitation. But that is a highly experimental proceedure in FW. DSB would be the same, and I am not anywhere near ready to try it - DSBs are designed to do to full anoxia and that would bother me.
Thanks for digging that up and thank you especially RTR. While on the subject, RTR, what would you consider a maximum safe depth for a sand bed assuming a medium-coarse silica sand?
Hebdizzle
07-23-2003, 7:45 PM
I know you didn't ask me but... I would say anything deeper than 3" is asking for trouble. :D
I was curious because I recently set up a tank with a sand substrate. Mine is about one to two inches with a light coating of fine gravel. I have about fifteen java ferns (55 gal) and two "eartheaters" who sift through the sand constantly.
Thanks for the link, that was very informative, but as RTR said seemed unresolved. Can you please define plenum, anoxic
Try ChilDawgs Fish Glossary Project (http://hometown.aol.com/mattgic/myhomepage/index.html) for those terms.
They are not there. I think this will help me understand better. I'll try google.
\Ple"num\, n. [L., fr. plenus full.] That state in which every part of space is supposed to be full of matter; -- opposed to vacuum. --G. Francis.
an·ox·i·a
Absence of oxygen.
For anyone whose vocab is as poor as mine.
I get it! Thanks RTR.
The distinction between anaerobic - lacking oxygen, but not absolutely without oxygen, and anoxic - without oxygen, absolutely, is key here. Denitrification is anaerobic while H2S and methane production are anoxic processes.
Plenum in tank terms is a water-filled space beneath the substrate in a tank (most often SW, but some FW). The plenum is created with PVC pipe sections supporting eggcrate plastic grid covered in turn with fiberglass screen to hold up the substrate. The water-filled plenum space provides extra diffusion space to avoid localized profound anoxia, maintaining anaerobic conditions without going all the way down. All substrates are anaerobic other than the most shallow and coarsest. The finer the substrate, the steeper the slope of decreasing O2 tension with depth - meaning sand gets more anaerobic faster than fine gravel, and fine gravel faster then coarse gravel. In planted tanks, anaerobic is good - most aquatic substrates are at least anaerobic. - that is normal root situation.
BTW, I don't do substrates greater than 1" unless it is RFUG, planted (then 3-6"), or plenum (all under test are 4" perhaps plus a tad locally).
I can't make a DSB suggestion, I haven't played with those. If I were going to test such, I'd mimic SW usage to start. Perhaps in a sump in case it caused problems.
So my basic understanding is this: In sw there is a gap where the water fills combined with detritus eating creatures rooting through the sand to ensure that the substrate is anaerobic and processes nitrates while a standard fresh water sand bed can become anoxic and then processes H2S and methane once the nitrates are used up. If these gasses escape everyone dies. Close? Yes, no, maybe? But I thought you also said sw beds are designed to do full anoxia?
DSB of sufficient depth for the grain size will go to full anoxia in either water condition. The gases generated in the anoxic levels should do no harm in an intact and undisturbed bed in either condition - as they migrate upward, they should be oxidized and rendered harlmess. (even the ammonia produced in denitrification will not escape the substrate, it is oxidized if it migrates upward. N2 gas is not, there are not enogh nitrogen fixing bacteria in FW or apparently in SW substrates to attack it so it bubbles out - but is harmless). But if distrurbed, esecially massive disturbance, all bets are off - the situation in the earlier thread.
There are other potential issues in either water condition - fusion of the calcareous sand in SW plenums or DSB, excessive organic build-up in planted FW. But those are different from the gas generation/release issues.
wetmanNY
07-24-2003, 12:07 PM
This is interesting material, and RTR always makes it richer and clearer.
Let me throw in two perceptions, ready for correction-- like my former misunderstanding about ammonium/ammonia conversion in the linked thread.
1. There's no value to a substrate deeper than what the rootzone requires. And aquatic plant roots tend to spread into a mat rather than delve deep into anoxic zones.
2. Troubles come not so much from the naturally low oxygen levels-- hypoxic to anoxic-- as from the biodegradable "enrichments" down there. A good way to get high cation exchange capacity, beneficial to plant roots, without adding organics to decay and release H2S and stuff, is with lateritic clays, either baked or unbaked.
WetnamNY:
1) You already know my feelings & experience re rootzones - in every tank I've planted it has been the entire depth of the substrate, however deep that is on the particular tank. I have broken down tanks where the bottom glass looked like it had a fitted mat - all root. Very like a decade-old houseplant which has not been repotted. I haven't yet had a substrate in a tank deeper than the rootzone. Even my all-Anubias plenum tank has roots in the plenum- not a mat, but roots.
2) IME & IMHO, CEC is good but limits are undefined on either end. There are periods when folks get CEC-intense, but nobdy has shown exacty what range is good or bad. Gravel, especially coated gravel, is poor, but older tanks with gravel grow plants just fine. The mulm (organic and inorganic) fills in quite well to a point. Excess organics are bad when deep in the substrate, and old dead plant roots can produce that over time. The time frame involved depends on the particular plant cultivar. Vigorous crypts take ~3 years IME, swordplants about the same, Val (even Jungle) may not take as long but does not ever seem heavy enough to produce the notorious anoxic pockets, etc.
Originally posted by RTR
DSB of sufficient depth for the grain size will go to full anoxia in either water condition. The gases generated in the anoxic levels should do no harm in an intact and undisturbed bed in either condition - as they migrate upward, they should be oxidized and rendered harlmess. (even the ammonia produced in denitrification will not escape the substrate, it is oxidized if it migrates upward. N2 gas is not, there are not enogh nitrogen fixing bacteria in FW or apparently in SW substrates to attack it so it bubbles out - but is harmless). But if distrurbed, esecially massive disturbance, all bets are off - the situation in the earlier thread. Just a thought as to how the mystery gas release may have happened in the other chaps tank. Perhaps a buried rock, driftwood or other ornament prevented the gas from leaching out gradually as described above. It built to the point that it released all at once and then caused the catastrophe. Just a thought.
Thank you for taking the time to explain.