Seachem Flourite vs UNS Controsoil: Which Substrate to Choose?

What we compared

Substrate is the hardest piece of planted-tank equipment to change later, so we compared these on the points that are difficult to undo:

• Effect on water chemistry. pH and hardness, and whether the substrate changes them.
• Ammonia and cycling. What the makers say about ammonia release and startup.
• Nutrients. Whether plants can rely on the substrate or need fertilizer.
• Preparation. Rinsing, cloudiness, and filling the tank.
• Lifespan and handling. Whether the bed keeps its structure, and what grain and color options exist.

The Flourite family shares the same base material in different colors and grades, so we include the standard, Black, Dark, Red and Black Sand versions. All facts come from Seachem's and UNS's product pages, listed in the sources. We have not tested these substrates hands-on.

Side by side

Inert versus active, in plain terms

Flourite is described by Seachem as a specially fracted, stable porous clay gravel that is not chemically coated or treated. Seachem states it will not alter pH, will not soften or decompose, and is good for the life of the aquarium. Your tap water's pH and hardness stay in charge.

Controsoil is described by UNS as a natural blend of soil and volcanic ash. UNS states it buffers water to a slightly acidic pH of about 6.8 over time and lowers total hardness. The substrate takes part in your water chemistry, which is the point of an active soil.

Neither is right or wrong. A soft-water shrimp tank and a tank of hard-water livebearers want opposite things. Our substrate guide covers how to match substrate to livestock.

Choose Flourite if

• You want stable water chemistry. Seachem states Flourite will not alter pH, so the substrate will not pull your water toward acidic or soft.
• You keep livestock that prefers harder or alkaline water. Because Flourite does not soften water, it avoids the conflict an acid-buffering soil would create.
• You want a substrate you never replace. Seachem says it is good for the life of the aquarium and will not soften or decompose, so the bed keeps its structure through replanting.
• You want color or grain choices. The family covers standard, Black, Dark and Red gravels plus a sand grade in Black Sand, in 3.5 kg and 7 kg bags.
• You use or might use an under-gravel filter. Seachem lists Flourite and Flourite Black as compatible.

Choose Controsoil if

• You want soft, slightly acidic water. UNS states it buffers to about pH 6.8 and lowers hardness, which suits fish and shrimp that prefer those conditions.
• You want to skip rinsing. UNS states no rinsing is required.
• You are wary of the large ammonia release some rich soils produce. UNS describes Controsoil as low in ammonia release because it does not contain high nutrient levels, and states tanks can cycle in under 2 weeks.
• You want a choice of grain. UNS offers Normal, Fine and Extra Fine. Normal is described as the standard for larger aquariums and plants with thicker roots.
• You want bag sizes keyed to tank size. UNS lists 1 L for about a 2 gal tank, 3 L for 9 gal, and 10 L for 20 gal.

When neither is right

• You want a nutrient-rich soil that feeds plants on its own. Neither is sold that way. Flourite is not described as nutrient-enriched, and Controsoil is low in nutrients by design. See other options on the Substrates: what to check before you buy ↗ hub.
• Your source water is very hard and you do not want to fight it. An acid-buffering soil works against hard water, and Flourite will not soften it. Consider whether your plant and livestock choices suit your water as it is.
• You only keep epiphytes such as anubias and java fern on wood and rock. These plants do not root into the substrate, so a plain gravel or sand may be all you need. Our easy plants guide lists examples.
• Your existing substrate still works. Replacing a substrate means tearing down the tank. Root tabs can often fix poor root feeding in an established bed.

Compatibility and maintenance differences

Setup and cloudiness. Seachem says Flourite is pre-washed but should be rinsed to remove dust, and that some versions, notably Black Sand, may need several rinses. Initial cloudiness is normal; Seachem suggests mechanical filtration such as filter floss or its Clarity product, and filling slowly, for example by pouring water into a bowl on the substrate. Controsoil needs no rinsing per UNS, though filling gently is sensible with any soft substrate.

Fertilizing. Both need help. Seachem notes laterite and similar gravel modifiers are unnecessary with Flourite, but heavy root feeders may still need root tabs. Controsoil's low nutrient content means you should plan on fertilizing from the start. Our fertilizer basics guide explains root tabs versus liquid dosing.

Cycling and water changes. UNS recommends weekly water changes to prevent ammonia spikes, which can be lethal to livestock. Seachem makes no ammonia claim for Flourite either way, since it is inert. With either substrate, cycle the tank fully and test before adding animals; see our cycling and water testing guide.

Long-term upkeep. Seachem states Flourite does not decompose and need not be replaced. UNS does not state a lifespan for Controsoil on its product page, so check your pH and hardness periodically to confirm the buffering effect is still doing what you expect.

Mixing. Seachem says Flourite Black is most effective used alone but may be mixed with other gravels, and Black Sand can be mixed with other gravels. Mixing an inert gravel with an active soil gives you less predictable buffering, so decide which behavior you want before you combine them.

Related guidance

• How to choose aquarium substrate
• Planted tank fertilizer basics
• Low-tech vs high-tech planted tanks
• Browse more options on the Substrates: what to check before you buy ↗ hub, or use the setup finder.