Fluval Stratum versus Bio Stratum
Two bags from the same company, made of the same Japanese volcanic soil, and the difference that matters is a number most listings bury.
We earn a commission if you buy through the links on this page, at no extra cost to you. It never changes a ranking, and there are products recommended here with no affiliate link behind them at all. Full disclosure.
What they have in common
Both are the same raw material: Fluval publishes both as natural, mineral-rich volcanic soil from Mount Aso in Japan. Both support a neutral to slightly acidic pH. Both are soils rather than gravels, which means both break down over years and neither should be rinsed hard or replanted carelessly.
If you were choosing between a Fluval soil and an inert substrate, that is a different and larger decision - aquasoil versus inert substrate works through it. This page assumes you have already decided on a soil.
The difference that decides it
| Fluval Stratum | Fluval Bio Stratum | |
|---|---|---|
| Also sold as | Plant and Shrimp Stratum | Volcanic Soil Bio Stratum |
| Grain size | 3-5 mm granules | Non-compacting 1 mm powder |
| Bacteria | Not stated | Infused with nitrifying bacteria |
| Material | Mount Aso volcanic soil | Mount Aso volcanic soil |
| pH effect | Neutral to slightly acidic | Neutral to slightly acidic; also softens water, reducing carbonate hardness |
| Shrimp | The one Fluval points shrimp keepers at | Grain too fine for shrimplets to shelter in |
| Bag sizes | 4.4 lb / 8.8 lb / 17.6 lb | 4.4 lb / 8.8 lb / 17.6 lb |
Everything useful on this page follows from row two. Three-to-five millimeter granules and one-millimeter powder behave completely differently once they are under water, and the plants and animals in your tank notice.
Why grain size decides a planted tank
Fine roots need fine substrate. A carpeting plant or a fine-stemmed species has roots a fraction of a millimeter across. Pushed into 3-5 mm granules those roots have nothing to grip - the plant floats back up within a day, which is the single most common complaint about planting into the original Stratum. One-millimeter powder holds them.
Coarse substrate gasses better. Larger granules leave larger gaps, which means more water movement through the bed and less chance of anaerobic pockets forming under a deep layer. Powder compacts more readily, which is exactly why Fluval describes the Bio Stratum grain as non-compacting - it is the property they have engineered around.
Bigger grains hide less debris. Uneaten food and detritus fall into the gaps between 3-5 mm granules and sit there until you vacuum. On a 1 mm bed, waste tends to stay on the surface where a siphon actually reaches it - which in a nano tank you vacuum weekly anyway.
Our plant picks are mostly rhizome species that attach to hardscape and need no substrate at all, plus two root feeders - the ranked nano plant list is worth reading before you buy either bag, because if your plan is anubias and java fern tied to wood then this whole comparison is moot.
Why grain size decides a shrimp tank
Newly hatched dwarf shrimp are a millimeter or two long and survive by being somewhere a fish is not. The gaps between 3-5 mm granules are shelter; a 1 mm bed is a flat surface.
That is why Fluval points shrimp keepers at the original Stratum, and it is why we would too. If your tank is a shrimp colony, the decision is made before you consider anything else on this page - buy the coarser one, add moss, and stop reading.
The bacteria claim, weighed honestly
Fluval publishes Bio Stratum as infused with beneficial nitrifying bacteria, and that is a real advantage on day one. What nobody publishes is a shelf life or a colony count - so a bag that has spent a summer in a warm warehouse is an unknown quantity. Treat the infusion as a head start on the cycle, not as a replacement for it, and confirm with a test kit rather than with the bag.
How much you actually need
Fluval publishes the formula and it applies to both bags: kilograms equals length times width times depth in centimeters, divided by 1000. Applied to the tanks this site covers, at the one-inch depth most nano scapes use:
| Tank footprint | At 1 inch deep | At 2 inches (for root feeders) |
|---|---|---|
| Fluval Spec V, 19.5 x 6.5 in | 2.1 kg / 4.6 lb | 4.2 kg / 9.3 lb |
| Standard 10-gallon, 20 x 10 in | 3.3 kg / 7.3 lb | 6.6 kg / 14.6 lb |
| Standard 20-gallon, 24 x 12 in | 4.7 kg / 10.4 lb | 9.4 kg / 20.7 lb |
Note what two inches costs you. Deep substrate for root feeders is a meaningful fraction of a nano tank’s height and volume - this site’s standing assumption is that substrate and the fill line take about 15% of rated volume, and a deep planted bed is how you reach the top of that range.
Filling a tank with powder without clouding it for a week
This is the practical complaint about Bio Stratum and it is entirely avoidable. One-millimeter grains go into suspension the instant a stream of water hits them.
- Do not rinse either soil. Rinsing washes the nutrients out and turns the bag to mud.
- Lay it dry, level it dry, and plant into it dry or barely damp.
- Fill onto a saucer, a plastic bag laid over the substrate, or the back of your hand - never straight onto the bed.
- Expect some haze for a day regardless. It clears. Do not run carbon to chase it.
Which to buy
Shrimp colony: the original Stratum. Not close.
Carpeting or fine-stemmed plants, no shrimp: Bio Stratum. The 1 mm grain is the reason those plants stay where you put them.
A betta tank with rhizome plants tied to hardscape: either, or neither. If nothing is rooting in the substrate you are buying soil for looks, and an inert sand or gravel costs less and lasts forever.
A mixed planted betta tank: the original Stratum. It plants crypts and swords adequately, it does not compact, and it leaves the door open to invertebrates later.
What we cannot tell you
How long either soil holds its nutrient load, or whether the Bio Stratum bacteria in the bag you receive are still alive. Fluval publishes neither figure, and both absences are recorded on the product entries rather than filled with a guess. We also have not run these side by side in identical tanks - this comparison is built from what the manufacturer publishes about each, which is the standard the whole site works to.
What we would buy for this
Chosen from the same registry as our roundups, on published specifications and the arithmetic they imply - not on testing we have not done.
Fluval Stratum
Volcanic soil from the foothills of Mount Aso, and the substrate we would put in a planted nano. It is a soil, not a gravel: it holds nutrients at the root, it supports a neutral to slightly acidic pH, and Fluval publishes the formula for how much of it you need, which almost nobody else does. It is also soft, light, and it will cloud if you dig in it.
Skip it if: You rearrange your scape often. Stratum breaks down under repeated disturbance and turns into a fine silt that clogs foam.
Fluval Bio Stratum
The same Mount Aso volcanic soil as Fluval Stratum, ground to 1 mm powder and infused with nitrifying bacteria. The grain size is the whole decision: 1 mm powder plants fine-rooted stems and carpets far better than the 3-5 mm original, and it is the wrong choice in a shrimp tank because shrimplets need gaps to hide in. Fluval publishes the bacteria claim; nobody publishes how long the bacteria stay viable in a sealed bag.
Skip it if: The tank is for shrimp. Fluval points shrimp keepers at the larger-grained original for exactly that reason.
Seachem Flourite Black Sand
A fired clay gravel, not a soil, and the only substrate in this list whose maker publishes an actual laboratory mineral analysis - iron at 4,338 mg/kg, run on a plasma emission spectrometer by the University of Georgia's chemical analysis lab. It does not alter pH, it never needs replacing, and it will not turn to mud under a replanting. It also will not feed a plant the way a soil does.
Skip it if: You want a substrate that buffers pH down for shrimp. Seachem states plainly that Flourite is not chemically coated or treated and will not alter pH.
Frequently asked
What is the difference between Fluval Stratum and Bio Stratum?
Grain size, mostly. Both are Mount Aso volcanic soil; the original is 3-5 mm granules and Bio Stratum is non-compacting 1 mm powder. Bio Stratum is also published as infused with nitrifying bacteria and as softening water.
Which Fluval substrate is better for shrimp?
The original Stratum. Its larger grain leaves gaps that newly hatched shrimp shelter in, and Fluval points shrimp keepers at it for that reason. A 1 mm powder bed gives shrimplets nowhere to hide.
Does Bio Stratum cycle a tank on its own?
No. The published bacterial infusion is a head start, and no shelf life or colony count is published for it. Cycle the tank and confirm with a test kit - a 2 ppm ammonia dose processing to zero ammonia and zero nitrite within 24 hours.
How much Stratum do I need for a 10-gallon?
On Fluval’s published formula, a standard 20 by 10 inch base at one inch deep is 3.3 kg - about 7.3 lb. Double it for a two-inch bed if you are planting root feeders.
Should I rinse Fluval Stratum before use?
No. Rinsing a soil washes out the nutrients you paid for and turns fine grades into mud. Lay it dry, plant into it dry, and fill slowly onto a saucer or your hand rather than straight onto the bed.
Where the figures on this page came from
- Aquarium Co-Op, Anubias Nana Petite - published light and CO2 requirements
- Aquarium Co-Op, Java Fern - published care requirements
- Fluval Stratum - published product page and substrate volume formula
- Seachem Flourite Black Sand - published mineral analysis
- Fluval Volcanic Soil Bio Stratum - published product listing