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Substrate Change


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Decided to remove some of the 'strate from my 90L planted. It's a dark, fine gravel, much like that Tahitian Moon sand stuff. It's been down for a year or so now, so is very well established. My idea is to syphon approx half the total volume out of the tank, right in the middle, leaving two largish islands of gravel to either end. The removed gravel will be replaced with a lighter coloured, coarse river sand as a contrast and feature.

How will my tank handle this do you think? I have read that when completely changing a 'strate, one must remove half at a time and leave a suitable time for the bacteria to migrate into the new 'strate. As stated the tank is 90L, inhabitants are a Paradise Fish, five Rummy nose and four Albino Cory's. Hardly a massive bio-load... It's also filtered by an AquaOne Aquis canister.

Thoughts??

...and yes, for the impatient among you, the old gravel will be used in my Barracks and pictures will be taken and posted... :alright:

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Interesting. My first reaction was, "why would it be a problem?". I suppose disturbing the gravel might allow a lot of noxious gunk to be released into the water column that your filter might not be able to handle. But if you're doing a water change at the same time I don't think this would be much of an issue.

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The problem would/might be in the sudden removal of 50% of the beneficial bacteria... Not too concerned with the clouding of the water at all...

I suspect all would be well, and I am prepared to do frequent changes, should a cycle occur because of the operation...

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But wouldn't the filter be doing most of the work? It'd be different if you were using an undergravel filter. I wouldn't have expected that the substrate would be doing that much in terms of filtration if water isn't actually moving through it.

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Yes, the bigger the filter, the more potential for greater colonies of bacteria...this is true, but the key word here is "potential".

There is only as much bacteria in a system as is required to handle a given bioload... If there is insufficient "food" (ammonia, nitrite/rate etc), the bacteria will not multiply. Should the bioload all of a sudden increase markedly, you will have an ammonia spike, which will last until the bacteria have multiplied sufficiently to handle the extra load. Reduce the load, and the bacteria die off to a level suitable for that load. Hence the admonition to only add fish to a system very slowly... (I apologise if that sounds a little patronising of your level of experience, Michael, but I have only recently got my head around this concept myself...I had always thought, bigger filter, no dramas...)

So, currently, between my filter, tank and substrate, I have just enough bacteria to support the ten fish I have. If I remove half the gravel, I am worried that this will cause an ammonia spike and a mini-cycle, until such time as the bacteria is repopulated.

At least that is what I think...

EDIT: With regards your mention of the gravel doing little in terms of filtration due to lack of water flow...the bacteria contained in the gravel is made up of aerobic and anaerobic varieties...that is, bacteria that rely on water/oxygen moving across them (just as the bacteria hosting on your filter media) and bacteria, deeper within the substrate, that do not require either element to survive. Both types contribute to the breakdown of harmful chemicals within the system...

Edited by Ghengis
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I may be wrong about this but I was under the impression that anaerobic bacteria only break down nitrates (by converting them to nitrogen gas). Ammonia and nitrites are converted by aerobic bacteria which would mostly reside in your filter. Of course disturbing the substrate could cause the anaerobic bacteria to be released into the water column and subsequently die, releasing ammonia in the process. I'm still of the opinion that the substrate isn't doing a great deal in comparison with the filter. Certainly not half the work. The anaerobic bacteria can only breakdown whatever makes it's way into the substrate by osmosis, whereas the filter is actually getting water constantly pushed through it. In terms of overall filtration I think more is happening in the filter than the substrate. I don't think removing half the gravel means that you are halving the ability of the whole system to cope with waste.

If I was planning on doing this I would remove the fish and as much of the water as I can store in buckets. Then scoop out the gravel trying to disturb what you want to leave behind as little as possible. Then put a bit of fresh water in and see how much muck appears. Maybe siphon out the muck and repeat this until you're happy, then put the old water back.

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Will be recycling the old water, for sure...I think performing this operation AND replacing the water might be asking for trouble! But rather than trying to scoop the gravel out, I am keen to try syphoning it, using just the hose portion of my gravel vac. I have used the hose in the past to target particularly obnoxious clumps of debris and it never has any trouble lifting out some of the gravel in the process, so I figure if I target just the gravel, I should be able to get most of it out without clouding the remaining water too much.

Thats the theory anyway...

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