Start with chemistry
Substrate is difficult to change after roots, hardscape, and livestock are in place. Choose the chemistry before the color.
A useful first split is:
- Buffering soil. Some soil substrates are designed to move water toward a particular pH or hardness range.
- Planted mineral substrate. These products provide a structured mineral bed but may not make the same buffering promise as a soil.
- Sand or gravel. These can anchor plants, but the material and source still determine whether a product changes water chemistry.
Do not use “active” and “inert” as complete product specifications. Read the exact manufacturer’s chemistry, preparation, and replacement guidance.
Fluval Stratum: a buffering soil
Fluval’s current Stratum page describes a volcanic soil with nutrients, minerals, and trace elements. It is designed to support neutral to slightly acidic pH and naturally reduce carbonate hardness. The standard granules are listed at 3 to 5 mm.
The effect is not a fixed pH number. Fluval’s support guidance says water with low buffering capacity can become more acidic, while hard, highly buffered water can remain alkaline. Test the source water and installed aquarium rather than buying Stratum to hit a precise number automatically.
Fluval also gives a general replacement recommendation of every 1 to 2 years, depending on the type and amount of plants, as nutrients deplete and the soil compacts. That variable schedule is part of the buying decision.
Eco-Complete is now Eco-Planted
CaribSea has renamed Eco-Complete Planted to Eco-Planted. CaribSea says it is the same product with the same product numbers, so older listings and searches may still use the Eco-Complete name.
This is not accurately described as blank, nutrient-free gravel. CaribSea’s current freshwater substrate page describes the black version as a basaltic volcanic base with iron, calcium, magnesium, potassium, sulfur, and other elements. It also includes porous grains and live heterotrophic bacteria. CaribSea publishes a broad 0.25 to 7 mm size range for the black version.
The current product page does not make the same neutral-to-slightly-acidic or KH-reduction claim that Fluval makes for Stratum. That makes Eco-Planted the clearer of these two when you do not want to choose Fluval’s explicit buffering role. It is still sensible to test the installed bed rather than promise no chemistry effect from any mineral material.
For a direct decision, see Fluval Stratum vs Eco-Complete. The best planted tank substrate guide adds plain sand and gravel to the choice.
A four-question substrate decision
1. What water do the livestock need?
Write down the target pH, KH, and GH for the actual species and source water. If a substrate is designed to lower KH or support more acidic water, use it only when that direction matches the target.
Substrate is not a replacement for appropriate source water, remineralization, acclimation, or testing. It also cannot hold pH at a fixed value when rocks, water changes, and additives are pushing the chemistry another way.
2. What must the bed hold?
Large rooted plants, fine carpets, removable stems, and hardscape-attached plants do not ask the same thing from a bed. Look at published grain size and structure, then decide whether the roots can be planted without constant uprooting.
Fluval describes Stratum as lightweight and non-compacting, but its FAQ acknowledges that fine-rooted plants can be difficult to secure and suggests a finer Bio-Stratum layer or a dry start. Eco-Planted Black has a much wider published grain range, so expect more texture variation rather than assuming every grain matches the catalog midpoint.
3. Where will nutrients come from?
Manufacturer claims about minerals do not prove that a substrate supplies every nutrient in the ratio every plant needs. Rooted plants may benefit from nutrients near their roots. Stems and attached plants can also use nutrients in the water column.
Choose fertilizer and root supplements from the plant mix, visible growth, measured water, and product instructions. Avoid a universal weekly dose or fixed root-tab calendar without knowing the fertilizer strength and aquarium conditions.
4. What will renewal look like?
Soil can compact and lose the characteristics that led you to buy it. Fluval publishes a variable 1 to 2 year replacement recommendation for Stratum. CaribSea’s current Eco-Planted pages do not publish a fixed replacement interval.
No published interval does not prove indefinite service. Layout changes, mixed layers, accumulated debris, and changing livestock can still make a substrate replacement worthwhile.
Depth and quantity
Choose bed depth from the roots and aquascape, not tank gallons alone. A shallow foreground and a deeper rear slope use a different amount from a flat bed in the same aquarium.
Fluval publishes a Stratum quantity formula: tank length × width × desired substrate depth in centimeters, divided by 1,000, gives an estimated weight in kilograms. For another product, use its own density or calculator rather than applying Fluval’s soil formula.
Map the planted zones before buying. Hardscape displaces substrate, retaining walls can hold a slope, and attached plants may need little or no bed beneath them.
Rinsing and cleaning are product-specific
Old rules of thumb are unreliable here.
- Stratum: Fluval recommends gently rinsing it in a large colander without agitation to remove fine dust. Once installed, hover a small gravel vacuum over the surface rather than cleaning it like gravel.
- Eco-Planted: CaribSea’s rinsing FAQ specifically excludes Eco-Planted from the products that should be rinsed.
- Sand or gravel: follow the exact bag. Material sold for a different use may contain dust, minerals, or contaminants that aquarium instructions would have addressed.
Think carefully before mixing or capping
Layering is hard to undo and the two manufacturers give different limits.
Fluval recommends mixing Stratum only with other soil substrates, such as Bio-Stratum. It warns that other substrate types can make Stratum break down prematurely.
CaribSea allows other substrates with Eco-Planted but suggests no more than 50 percent. CaribSea says a cap belongs only in a new setup, and warns that covering Eco-Planted can reduce early biological filtration and lengthen the cycle.
For that reason, a Stratum and Eco-Planted sandwich is not the default recommendation. Pick one main bed unless the manufacturers confirm the exact layer plan.
Common substrate mistakes
- Choosing a pH-buffering soil without measuring source-water KH.
- Calling every mineral substrate inert or every soil identical.
- Copying rinse directions from a different product.
- Buying by gallons without calculating footprint and desired depth.
- Burying a plant’s rhizome when it should remain above the bed.
- Expecting substrate to compensate for unsuitable light, unstable CO2, or missing water-column nutrients.
- Adding livestock before pH, hardness, ammonia, and the nitrogen cycle are ready.
- Assuming bottom-dwelling fish are a substitute for appropriate cleaning.