Sand, Bacteria, Gravel.
I've read a number of times, here and elsewhere, that sand is inferior to gravel due to it's compacting nature with regards to the bacteria cultures, specifically the nitrifying kind that eat the ammonia in the tank.
I'll ignore the benefit of plants which negates this argument altogether... so assume that plants are not a factor and this would be for a cycled tank that relies on the bacteria for ammonia handling.
A few widely accepted points regarding sand substrates (I hesitate to call them facts at this point but they are acknowledged on some level generally as such)
- nitrifying bacterial culture in tanks develops and lives only on surfaces in the tank, decoration, plants, filter media and substrate
- sand tends to compact and is useless in regards to nitrifying bacteria growth and efficacy
- the top 1" is aerated and can support aerobic bacteria
- ammonia producing waste tends to settle on the surface of the sand
- ammonia producing waste is easily removed without the need for stirring up the substrate
The second and third points seem to contradict each other somewhat. The third is more correct even though I have found that the sand doesn't actually compact much until deeper than that so I'll go with a 1" deep bacterial support as that is more widely accepted than number two. In my case, I employ trumpet snails and I regularly mess with the plants so the sand doesn't get a chance to compact even though I don't think that it does as much as some think.
A few widely accepted points regarding gravel substrates.
- the larger grain size provides water flow through the substrate promoting aeration and bacteria growth throughout the gravel.
- lack of compaction allows more ammonia producing waste to sink into the gravel
- lack of compaction allows cleaning of the gravel fairly easily removing ammonia producing waste
Looking at gravel and assuming an average grain size of 1/8" (I've seen larger and smaller, but this is just for the math) and using the average between surface area of a sphere and a cube of this size (good luck actually measuring the surface area of such an irregular shape) of 0.072 square inches (approximately 1/14 square inches) and 512 grains per cubic inch that makes 36.9 square inches of surface area for each cubic inch of substrate.
Doing the same thing for sand with a grain size of 1/32" (check out the pic, this number is not necessarily accurate, the increments are 1/32" on the ruler) produces a surface area of 147.5 square inches per cubic inch of sand. There would be 32,768 grains. I think that this is low as the grain size I used looks to be smaller, on average, than the 1/32".
Now, I'll use my aquarium for sizing at 30" x 12" for a bottom surface area of 360".
Assume 2" of gravel (some may be deeper or shallower, I see it both ways in tank shots) that makes 720 cubic inches of substrate at 36.9 square inches of surfaces per cubic inch which adds up to 26,568 square inches of surfaces for bacteria to cling to plus the other tank surfaces. That is the equivalent of about a 13.5' x 13.5' surface.
Assume only 1" of sand as that is the most widely accepted aerobic zone no matter the actual depth, the surface area yield is 53,100 square inches, again plus the same additional tank surfaces. That is the equivalent of about a 19' x 19' surface.
The larger the gravel grain size and/or the smaller the sand grain size (of course clay is a different story, so there is a too small) the larger the discrepancy between the two media surface areas.
Basically, sand would appear to be a better substrate based solely on the capability of holding nitrifying bacteria to deal with ammonia in the average tank. Couple this with the ease of removing the waste due it settling on the surface not to mention the comfort of many bottom feeders preferring sand over gravel and the cheap cost of the sand (assuming some sort of play sand here) I don't know why anyone would choose gravel over sand.
Well, maybe colours and the fact that there is a more stringent cleaning process required before using it. I happen to like a more natural look so I would discount the colour as a factor altogether.
Thats a damn good post Jeff, and one that provokes quite a few questions, but one that i know zilch about :-)
I'll be following this with interest though as after changing the 15 gallon from gravel to sand i am considering still either going 1/3 sand 2/3 gravel OR all sand in my 55, especially as the gravel does get messy to say the least.
I was under the impression that sand offers no beneficial bacteria to the cycling process so if it does it would be more than thought provoking...
Sand is a surface just like any other. When you consider how small the bacteria are, the sand is not so small anymore... relatively speaking.
Impressions are often misleading without anything to back them up.
I read a fairly extensive article on sand used in marine (salt water) setups and different bacteria that they promote growing to deal with their similar issues and they count more than the top 1". Their surface areas, due to the tank size and depth that they count as beneficial, are up in the 1/3 acre sizes... that's a LOT of surface area. I think I referenced it in a previous post but I can't find it quickly now.
I read lots of great stuff and often I can't easily find it again... I've got to start cataloging my reading.
Oh, going gravel and sand gives you no benefit of the sand and all the downsides of the gravel as the gravel sits on top over time as the sand filters down between and sits on the bottom. So you would still have food and crap settling into the gravel and needing to vacuum it rigorously, smaller surface area media at the top of the substrate where it counts the most and sand down in the, assumed, compaction zone.
I'd recommend just picking one and sticking with it... and you know it would be sand that I would recommend.:roll:
BTW, I'm not bashing gravel or suggesting that it won't work, of course it works... I feel that sand sometimes get s a bad rap even though many here use it and recommend it... it only takes one naysayer to change someone's mind on a good thing even when there are so many saying good things. I find that people often vehemently support a negative position without any real background or research in the topic often just based on someone else's say so.
It would be a beach and cliff thing with the sand and gravel seperated. Lets see if i can draw it here as i have time on my hands... haha
G = gravel, P = Partition, S = Sand.
gravel at 5/6 cm, sand at approx 2.5, clearly separated by partition.
haha, lots of free time.
If you find that article on they sand, feel free to PM me with it :-) Will ask though, as you said it relates to marine, can this be transfered over to FW? As in is the bacteria different etc?
I've got sand in all my tank's presently, and have never worried bout it unless deeper than two or three inches ,plant's or no plant's.
Raised a group of Discus about the size of a half dollar, to adult size over sand substrate, and other than vaccuming the surface each week,, I gave it no extra care sifting,etc.(did employ trumpet snail's)
I would go further, although not recommend it to new hobbyist's,,that there is little need for vaccuming any substrate so long as your not grossly overfeeding/overstocking.
Fact is, most/many DO overfeed /overstock, and so vaccuming substrate becomes of great benefit to the fishes by helping remove organic's.Not many types of filtration will remove excess debri,organic's from the substrate once they fall there.
The trouble with the gravel, to my thinking, is that food that drops to the bottom is no longer available to the fish and in order to ensure that the fish actually get enough, you almost need to overfeed... which leads to needing to vacuum the gravel for the leftovers.
Takes some trial and error to feed fishes without overfeeding and witnessing the uneaten food on substrate.
Is more difficult in my view to feed one or two fish, than it is to feed many.Fishes will instinctively learn to forage along the bottom once they realize that food's will not be offered several times a day.
Have witnessed even those fish who prefer to feed from the surface, forage along the bottom, and have also observed those fishes fed several times a day,,let food's fall to the bottom for they know that more will be comming later.They seem to get particular with the food's associated with several feeding's, and are much more aggressive when feeding's are but once a day or every other day which also help's with water quality.
Many folk's sprinkle excess amount's of food over the surface, rather than feeding a small amount,,waiting until it's consumed,and feeding a little bit more (correct way in my view).
Sand substrate does keep food's ,waste,on the surface for easier cleanup but some folk's go about cleaning the sand in wrong way by poking,sifting,while vaccuming which can/does,,allow that which you want to remove getting buried in the sand,gravel.
Best to vaccum up what is visible,and THEN,poke or sift the sand,gravel.Course if you don't offer too much food,and tank is not grossly overstocked,then much less need for vaccuming the substrate with few exception's.No way around vaccuming in over stocked tank's or tank's holding large waste producing fish in my view.
Plant's provide much benefit also by using waste,excess food's, for growth. But plant mass must be in proportion to waste being created.8-)
I've only ever heard one negative comment about sand (and I participate in about 3 tropical fish related forums). This comment came from the gal at my local Petsmart who generally really knows her stuff. She told a customer that sand was no good for plants because it compacts too much. Of course, she was incorrect as not all sand is created equally. In all fairness, sand, like gravel comes in many particle sizes. It's likely that very fine grained sand, especially when deeper, could compact and create problems. However, many/most well rinsed/washed sands probably do not compact at avg. aquarium depths of 1-3" or so.
In my case, I'm using pool filter sand that is screened to provide particle sizes that water will easily pass through.
Also, when we speak of surface area, compared to gravel, sand has more. However, in any tank the size of a beneficial bacteria colony will be relative to the amount of nitrogenous waste and the degree to which the tank is planted and/or has floating plants. So substrate surface area becomes nearly a moot point, especially when we also factor in the filter(s).
I will say that having used both now, I'm a fan of sand over gravel as it is much more maintenance free. Uneaten food and any detritus just remains on top, making tank management much easier. With my two Pepper Corys, there is very little substrate effort required on my part (with gravel I would need routine gravel vacuuming.)
such an informative thread!:thankyou:
Indeed it is an informative thread.
AbbysDad's comment about the efficacy of sand vs. filter media in harboring nitrifying bacteria, reminds me to ask a question which has intrigued me ever since I cycled my first bare-bottom tank.
JDM's interesting mathematical exposition of the surface area of sand vs gravel paints only one part of the picture. Beside surface area, which effects colony size, bacteria also need current/flow to provide the food (ammonia) for reproduction. The bacteria inhabiting the filter is awash in food, while that in the sand is subjected to a much lesser flow implying a meager diet and lower efficiency.
Is a few cubic inches of dense filter media, even though it contains a smaller colony, relatively more efficient than a much larger amount of sand? At what relative sizes would they be equally efficient?
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