Originally Posted by Blue Ridge Reef
Agree. I think the abrupt pH raise could very well kill the "stain" of living organisms on your ceramic rings.
Only that I think that the filter was in bypass when cement dropped into the water… (I could not swear it, but it is what I recall). Anyway if not, this could add to what both, Amp2020 and Surf say.
Here's my guess. The crud was held on by living bacteria / slime. That layer of biofilm makes it hard sometimes to clean media. Bleaching it breaks the bonds and makes it easier to rinse the organics away when cleaning. In the filter, once all the bacteria died from lack of flow and oxygen. It released its grip from the media and easily washed away.
When some time ago, I thought I had bacteria dead (filter 24h stopped, and I cleaned it with tap water -I knew I killed the bacteria that may have been still live-) I could not clean at all the crud, so I am not sure about this but maybe some time or/and biological action is needed before the bacteria looses and the organics could be removed/rinsed away. Even so, I suspect the crud might be 'protected' from scrubber or washing by the micro-cavities… On the other hand, is it so hard the biofilm, as to protect the crud from scrubbers? Maybe yes.
But on the other hand what you say could make sense theoretically, and in fact, it could be the explanation why this 'crud cleaning/removal' does not work when filter is running normally. I have been thinking and I suspect that maybe the driver has not been depleted O2, as the water was saturated with it, but the NH4/NH3/NO2 needed to feed the bacteria/archaea which was -as it should- much less present than O2 (non measurable) in pond water: I think that the NH3/NO2 was depleted long before the O2, good colony died, O2 remained, thus as you say, then the bacteria released its grip from the media, and saprophytic aerobic
bacteria took over and reduced the DOM to NH4/3. This would explain why I had in the filter water, 10 mg/l of NH3, (when in pond water did not), and why there was no rotten egg smell present.
I personally think it is a mineral deposit. If your pnd is a little short on chlorides and sulfate. micro nutrients typically are sulfate or chloride. If those nutrients run low some of the micros may be converted to oxides and plate out on the media. which may result in a black, brown, or yellow mineral coating. however if the chlorides and sulfates concentration may increase dropping your PH a little (maybe not enough for you to notice) and that may cause the mineral deposits to dissolve.
Bleach is often used to help remove mineral deposits. The minerals convert to chlorides which dissolve easily.
That is a smart one, I did not think on that, I will keep this in mind. Regarding our case I am not sure: I can't remember what I could do to increase these concentrations (maybe if the cement has them, and the filter was not in bypass -which I think it was). Nevertheless, this would not explain the big increase in NH4/NH3 in the isolated filter water… Except if the DOM needed to produce -biologically- NH4/NH3 would come from other parts of the filter and the crud disappeared as you say…. But if the DOM used to produce NH3/4 comes, for example from sponges (not the ceramic media) then why it (I mean, massive production of NH3/4 and sponge/other media cleaning) did not work all the time and not only when filter is stopped?
Besides, the sponges had the normal (few) dirt for a week of filtering when I opened the filter…
Do cement increase sulphates and chlorides?
I think I could do a test when the ceramic media gets dirty once more. I have added bottom aeration to the filter. It will be normally closed (only used for backwash). I guess next time, having water oxygen saturated I could bypass the filter, let it sit for four days or so with aeration, and check if the dirt is gone after this time.