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I need @taricha to jog my memory of out many discussions and experiments on sand beds. In short, they are complicated ecosystems.
So sandbeds can be found to contain ammonia, but the amounts are usually too small to explain nuisance growth, and way way too small to create a tank-wide elevated ammonia level of concern.If you carefully remove water from the sand (pore water) without mixing it with water from above the sand bed, the water between the sand grains contains ammonia and a higher concentration of phosphate. Dissolved oxygen is definitely lower by several ppm. i think the alkalinity and pH are also different from the water above the sand bed. The total amount of pore water compered to the total amount of aquarium water is small and alone would not cause a large ammonia spike. A sudden release and subsequent digestion of organic matter from the sand when stirred up might cause ammonia to rise (I have not done this).
I found that I could test porewater and get stuff like 0.2 to 0.4 ppm total ammonia higher than the zero in the tankwater. pH in my porewater is always at the aragonite buffer of 7.5-7.6 because decomposing organics in very low flow lowers the pH with CO2 until the sand buffers it. The oxygen level is zero at some depth (some sulfide smell can appear), and above zero in the upper levels. Phosphate (and Fe) in my old sandbed was significant. PO4 of 0.10-0.15 in the water, but 1-1.5ppm PO4 in the porewater. Fe was found to be present in the fine particle dust in my sand at nearly the same levels expected in living cyano cells.
I tried squeezing N out of old sandbed grunge every imaginable way (including some heated acidic digestions), looking for enough N that could drive nuisance growth and eventually came to the conclusion that it's squeezing blood from a stone. If you work hard enough, you can measure some N but the amounts are so tiny you'll never get good nuisance growth off of that N. But P and Fe? plenty there to sustain any organism capable of exploiting it.Yeah, the Seneye would be a device to detect ammonia in the sand. Stirring the sand and catching a whiff of ammonia might work. The “might” qualification is based on not mixing too much above sand water with the pore water. The merit in this idea for me is that the sand bed as an ammonia generator could be part of or the entire explanation for why nuisance algae grow immense populations on sand
I picture their appetites like those of my dogs: insatiable. If more food becomes available, the more of them there are. The aquarium sand bed probably does not have the intensity of organic matter raining down on it like that in the ocean or on a reef.
Agree - I simply don't see any pathway nor evidence of long term storage/accumulation of large amounts of N.
The exceptions prove the rule here: a big fish like to poop in one corner and cyano grows there, or a pile of dead GHA slimes over with cyano are the sort of clear organic N inputs you need to drive good nuisance growth with old sand.
Not just my sand is 10 years old so there's years of fish food/poop nitrogen stored in there. That stuff has been exploited long ago.
In a light driven system (which is every reef tank) - I estimate the end state is probably around 90% ammonia consumption by photosynthetic stuff, and <10% by nitrifiers. This was the case in my sandbed and system, and sand I took from some LFS had roughly the same nitrifying capacity as my own.. @taricha I think has done some experiments to tease out the proportions of ammonia processed via photosynthetic organisms and nitrifyers - or at least had some clever ideas on the subject. It’s been years since.
We can think about some failure modes that might apply here.Yes, old rocks might be well populated with ammonia consumers after ten years, In an aquarium where water flow is relatively poor, the rock surfaces might have thick biofilms that might act as a barrier to bringing ammonia and CO2. Biofilms are stick and all sorts of stuff sticks to them, adding more resistance to the influx of ammonia. Sometimes old isn’t so good
Old sand kept and re-used. It's unlikely that the same surfaces that were exposed before and thus had all the nitrifying biofilm are still exposed now in the new sytem. biofilms aren't evenly distributed in the sand and most of the nitrifiers got buried.
similarly but not as extreme with the rocks. Nitrifiers likely seek out the best surfaces on rocks and concentrate there, with the best flow of aerated water across the surface. pick up all the rocks and put them in a new system. Maybe the "good" surfaces don't get good flow anymore. This is very temporary - biofilms will adjust in a few days.
But what if your system is 90% ammonia uptake from photosynthetic organisms like corals and anemones. And you move them all over to a new system and something about the change has them really ticked off. What's their uptake now? I bet a lot smaller. Maybe zero. Maybe even negative, if they close up tight and squeeze out some chemicals in distress, then your ammonia consumers may now be releasing nitrogen
Add in the fact that all those organisms will release slime = digestible organic carbon. Add in that some dead anemones and urchin have been found in the sand. So you have lots of decaying organic matter. The decaying organic matter in addition to releasing some N, is also releasing a lot of digestible organic carbon. This lowers the O2 level. You don't have to push O2 to zero to really slow down nitrification, you can probably slow it way down at 30-50% of max O2.
If the nitrifiers are fighting with heterotrophs for oxygen in an environment with lots of organics floating around, they aren't gonna win that. Heterotrophs do. Also the skimmer has been offline for some of this time.
If the majority of previous ammonia uptake was from softies and nems that are now really ticked off, they probably aren't doing the job either.
So who exactly is supposed to be consuming all the ammonia now? maybe nobody can in this little (mini-cycle) scenario.