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The choice, in one paragraph
An active soil like Fluval Stratum feeds plants at the root and supports neutral-to-slightly-acidic conditions, which suits plants and shrimp. It is soft, and it breaks down over a few years, particularly if you replant often.
A fired clay gravel like Seachem Flourite never breaks down, never needs replacing, and does not alter pH. It also does not feed anything, so rooted plants want root tabs.
If your tank is planted and includes shrimp, take the soil. If you expect to rescape repeatedly over years, take the gravel.
How much you need, from published formulas
Fluval publishes an actual formula for Stratum: kilograms = length × width × depth in centimeters, divided by 1000. Almost nothing else in this category tells you how much to buy.
Applied to common nano footprints at one inch deep:
| Tank | Footprint | Substrate at 1" | Bag to buy |
|---|
| Fluval Spec V | 19.5" x 6.5" | 2.1 kg / 4.6 lb | One 4.4 lb bag |
| Standard 10-gallon | 20" x 10" | 3.3 kg / 7.3 lb | One 8.8 lb bag |
| Standard 20-gallon long | 30" x 12" | 5.9 kg / 13 lb | One 17.6 lb bag |
Add roughly half again if you want a two-inch bed at the back for planting depth, which most scapes want.
What Seachem publishes about Flourite, and why it is unusual
Seachem prints a full mineral analysis for Flourite Black Sand - iron at 4,338 mg/kg, calcium at 29,950, aluminum at 33,150 - measured on a plasma emission spectrometer by EPA Method 6010 at the University of Georgia’s Chemical Analysis Lab.
We have no way to verify that ourselves and we are not claiming to. What we can say is that publishing a third-party laboratory analysis with the method named is a standard of disclosure nothing else in this hub comes close to, and on a site that ranks partly on what manufacturers are willing to put in writing, it counts.
Grain size and depth
Depth: one inch, two at the back. Deeper than about two inches in a small tank creates anaerobic pockets. A slight slope up toward the back creates the illusion of depth in a shallow tank, which matters in a nano.
Grain: fine to medium, and smooth. Sharp aquarium gravel is a hazard for anything that grubs in the substrate. Very fine sand compacts. Neither Fluval nor Seachem publishes a numeric grain range for these products, which we record as an absence in both specification tables - it is the figure a nano keeper most wants and neither provides it.
On black substrate
Both picks here are dark, and that is not only aesthetics. Fish held over a pale substrate tend to wash out; over a dark one, color reads stronger. Whether that is comfort or camouflage response we are not going to speculate about - but the visual difference on a betta is obvious and it costs nothing to choose.
What about bare-bottom?
A legitimate choice, and the easiest tank there is to clean - waste sits visible on the glass and siphons off in seconds. It is standard practice in breeding and quarantine tanks for exactly that reason.
What you lose is a large amount of surface for bacteria, the ability to plant anything rooted, and the visual difference is stark. If you go bare-bottom, lean hard on rhizome plants tied to hardscape, which is exactly the situation they suit.
Setting it up
Rinse a clay gravel until the water runs clear - Seachem states Flourite is pre-washed and may still need it. Rinse a soil gently or not at all; Fluval says to rinse Stratum without agitating it, because agitation breaks the grains down.
Then fill onto a plate or a plastic bag laid on the substrate, never straight onto it. The full setup order.
