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Discussion Starter · #1 ·
Ever since I have been keeping a planted tank, about 20 years, it has been the consensus that the substrate should be at least 1.5 to 2 inches in depth. I have not seen anything to convince me that that is bad advice. But, has anyone done enough experimenting, either on purpose or accidental, to find out what happens when you don't have sufficient substrate depth? What is the effect on the plants? When troubleshooting, what symptoms go with inadequate substrate depth? This is for rooted stem plants, primarily, but rooted plants in general too.
 

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Hi Hoppy,

I started with aquariums as a kid. Before the invention of undergravel filters (UGF) (i.e. Eureka Brand/ tubes with holes or Miracle Brand / flat panels) I believe the consensus was just enough substrate (gravel back then) for the plants to root and stay in place. When undergravel filters became popular in the mid 1960's I was working in an aquarium shop / fish wholesaler operation and 1-1/2" to 2" of substrate was recommended to provide enough gravel media for a large enough colony of the beneficial bacteria to handle the nitrogenous waste of the fish and any excess food. I think that the we never went away from that 1-1/2" to 2" recommendation even as the popularity of undergravel filters decreased and HOB and canister filters became more of the standard for filtration.

That said, I have two planted tanks that I still run undergravel filters in (with 2"-3" of gravel) and the plants do fine; I do also have HOB or canister filters on the tanks. The UGF filter tanks typically 'clear' much quicker than my HOB/canister filtered tanks when dosing calcium sulfate (which makes the water milky). I also have 2"-3" of substrate in my non-UGF tanks partly due to some of my stem plants (i.e. Bacopa madagarensis) being very buoyant and I need to plant the stems 'deep'. Also I use calcined clay substrates in all my tanks and it is a 'light' substrate (about the same as ADA Aquasoils) and planting deep helps hold plants and stems in place when the corydoras are doing their cleaning. If I used a heavier substrate like blasting grit I might try to get by with a thinner layer of substrate.

Just my thoughts.....

45 gallon tall *24" high) with UGF, HOB, and Canister filters (tube of UGF right-rear)
 

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In nature you don't see plants growing in shallow sediments very much (in this case an inch or less on to of ledge rock or boulders) and i would imagine it's due to currents and water movement preventing plants for maintaining a firm hold on the bottom. I would also expect that those plants that are in more water movement probe areas actually slowly crept to that location from less agitated areas rather than from advantageous fragments that landed there. Also the only exception I've found is clefts and cracks in rocks where plant roots can adhere.

I expect the same is true in aquariums. The more water movement you have the more substrate mass on a plants rid are needed to keep it in place. With that logic I would assume that in a tank with no water movement at all you could probably just have them sit there with no substrate at all as we step up our filtration we need more depth/weight. Stem plants will regularly put out rid way announce the substrate to feed from the water column, so nutrients aren't really an issue, is more for keeping then in place than anything.

Plant fragments well often float around and grow until they land on a spot with a lack of water movement sufficient for them to settle and sit there and develop roots that go into the substrate all by themselves no planting at all. Which leads me to believe substrate lays more of a nutrient sink (long term) and ballast than anything fantastically important for plant growth assuming nutrients are in the water column.

A quick test could be setup with stem plants and plant weights in a bare bottom tank (or go by the anecdotal evidence of lfs plants that survive and grow without being planted in substrate with weights)

Tldr my wandering thoughts...
Plants need enough substrate to stay put with given flow, no more, no less
 

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In nature you don't see plants growing in shallow sediments very much (in this case an inch or less on to of ledge rock or boulders) and i would imagine it's due to currents and water movement preventing plants for maintaining a firm hold on the bottom. I would also expect that those plants that are in more water movement probe areas actually slowly crept to that location from less agitated areas rather than from advantageous fragments that landed there. Also the only exception I've found is clefts and cracks in rocks where plant roots can adhere.

I expect the same is true in aquariums. The more water movement you have the more substrate mass on a plants rid are needed to keep it in place. With that logic I would assume that in a tank with no water movement at all you could probably just have them sit there with no substrate at all as we step up our filtration we need more depth/weight. Stem plants will regularly put out rid way announce the substrate to feed from the water column, so nutrients aren't really an issue, is more for keeping then in place than anything.

Plant fragments well often float around and grow until they land on a spot with a lack of water movement sufficient for them to settle and sit there and develop roots that go into the substrate all by themselves no planting at all. Which leads me to believe substrate lays more of a nutrient sink (long term) and ballast than anything fantastically important for plant growth assuming nutrients are in the water column.

A quick test could be setup with stem plants and plant weights in a bare bottom tank (or go by the anecdotal evidence of lfs plants that survive and grow without being planted in substrate with weights)

Tldr my wandering thoughts...
Plants need enough substrate to stay put with given flow, no more, no less
I second this. Proper flow in your planted aquarium is a must (to get CO2/nutrients distributed). If your plants are too shallow, even the slightest water flow might rip your plants out (unless you weight them down of course).
 

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I think holding them down is the primary purpose for this depth of substrate. Unlike pots or other confined spaced holders, the roots can still spread out horizontally along the bottom of the aquarium. Additionally, assuming proper current flow, the nutrient content of the soil will not be limited due to confined space since it is constantly being added.

I'm thinking about this from a potted plant perspective when you have a plant in a pot that is too small. In that case the roots can choke themselves and the plant does not get enough nutrients. However in aquariums the horizontal is not limited so the roots should have plenty of room to grow. The one exception may be carpeting plants where the density is high. This would mean the roots would primarily have to go down and may get choked by neighboring offshoots leading to patches that die off. I suspect deeper substrate would limit this or at least slow it down.
 

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I have a 10 gallon fry tank for guppies currently with .5 -1.0 inches of carib sea substrate. I only have swords in the tank and VERY inadequete lighting bc its only for fry, but I have gotten some runners with the swords. I actually have some sprouts now.
 

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<--Avatar piccy...

...from 1994, 40 gallon long with a mix of 1" to 1.5" inch thick, with no substrate amendments, consisting of rather coarse river gravel and aquarium gravel sold by the LFS. The plant behind the Bleeding Heart is a big Echinidorus Horemani ( I think they're now called E.Uraguaensis..) Big strap like leaves and was propagating with small offshoots. At the other end of the tank was some mixed Crypt Balansae and Wendtii forming their own little thicket, a forest of Hygro Temple, and gravel attached Java Fern, Moss.

Not my most productive tank, but it did well enough to require a bit of alternate monthly thinning. It also had a some under tank gravel heat from two, 10 gallon fry grow out tanks with incandescent lighting.

Larger image below
 

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Discussion Starter · #8 ·
These answers are about what I expected. Substrate depth seems to have very little effect on plant growth, if any effect. My 65 gallon tank currently has about 1-1.5 inches of ADA Aquasoil, but the Aquasoil has a limited life, as a nutrient provider. I'm thinking about rebuilding it with topsoil, for nutrients, topped with black blasting grit, and was figuring out how much of each I was going to need, when this question came to mind. So many of our "rules" were started so long ago, and just passed on since then. Many of those "rules" no longer apply, or are not nearly as important as they used to be considered to be. Substrate depth may fit in that category. Unlike fertilizing, substrate depth is a consideration only when you first set up your tank, so I don't think the subject gets much serious thought.
 

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As you can determine from my photo, the PFS substrate I've always used in my discus tanks has never been more than approx. 1/2" at the front in the open areas, and not more than 1.5" or so at the rear in the planted areas.

I do concede that discus tanks and the substrate used therein make for a bit of a different animal than most tropical fish set-ups, in the sense that one purposefully wants to keep only thin layers, or depth, of plant-growing medium for ease of cleaning & to maintain over tank cleanliness & good water quality (particularly if using gravel).
Having said that though, I've always been blessed with reasonably good plant development & growth in no more than 1.5" depth of substrate, and I use a lot of Echinodorous (swords) in my tanks.


 

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These answers are about what I expected. Substrate depth seems to have very little effect on plant growth, if any effect. My 65 gallon tank currently has about 1-1.5 inches of ADA Aquasoil, but the Aquasoil has a limited life, as a nutrient provider. I'm thinking about rebuilding it with topsoil, for nutrients, topped with black blasting grit, and was figuring out how much of each I was going to need, when this question came to mind. So many of our "rules" were started so long ago, and just passed on since then. Many of those "rules" no longer apply, or are not nearly as important as they used to be considered to be. Substrate depth may fit in that category. Unlike fertilizing, substrate depth is a consideration only when you first set up your tank, so I don't think the subject gets much serious thought.
I think the depth rule is still important even though it's an old one. It may not affect plant growth, but it is helpful to have some depth so that during maintenance your plants don't uproot easily and float away.

I did carpet some Monte Carlo in shallow (1" or less) substrate before, and it grew normally. It was just a bit annoying because during trimming some would uproot and random patches would result. It was also annoying to re-plant because it was so shallow. So I guess it would just save you a few hairs to have that depth.
 

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Off Topic: Hi discuspaul,
That is a very nice discus tank with beautiful healthy looking discus. Question - is that Nymphea micrantha in the foregound right?
 

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I recall seeing a picture of one of Tom Barr's tanks where he was growing some HC in about 0.5 inch of substrate. It wasn't that long ago I saw it either and he said they grew that way with no problem.
 

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The deeper your substrate is, the more likelihood there is of toxic anaerobic gas pockets developing (No oxygen present in the lower regions). It most often occurs in layers that are 3" or more in thickness, but occasionally it can also occur in depths of around than 2" or even less. I've never heard of any instance where this happens if the substrate is 1" thick or less because oxygen is able to reach through that low depth of substrate.
Play sand substrates are seemingly the main offenders due to their tendency to heavily compact.
 

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I have used four or five inches of substrate for year's without issues with plant's or fauna.
People keep regurgitating the same old same old with respect to the hydrogen sulfide toxic killer, and take no time to actually explore the science of what happens when the gas makes contact with O2 in the tank, and how plant's can transport oxygen to the area's around their root's making the sulfide gas even less of a concern.
If it were truly as dangerous as some still preach,then those running deep sand bed's in marine tanks would report the danger's considering the $$ they spend on fishes.
I ain't sayin the gas does not form in deeper substrates, just that it is not the boogey man that many think it is.
Only time I smell the gas in my deeper substrates is when I go rooting around and pulling up plant's, or when I tear down the tank.
Nobody I know, or you know , has ever been able to point definitively to this gas escaping as root cause for issues with fauna or plant's.
Only speculation,grasping, for somewhere to lay blame.
 

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I have a new 10 gallon dirted tank with MCGO and eco-complete its above 3.5 inch.

I am having a lot of air build up currently, poking the substrate is not doing any good, the excess air is messing up the tank, the dirt is coming up. This weekend I am going to do compress (with something flat or hand) the hell out of the substrate and do multiple water changes. I wish I should have gone light on the dirted substrate.

I have above 15 malaysian trumpet snails also, I dont think its gonna help much at this level of build up.


I love my 5 gallon which I have used the left over of the eco-complete, so the depth is less than 1 inch. Its a Dwarf Hair Grass only tank and a java moss wall.

None of the tanks have fish, so I can go wild on water changes else it would have been a disaster.
 
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