Sand Substrate
Fine grains, 0.5-1mm. Perfect for bottom-dwellers (cory, kuhli, stingray, goldfish), biotope tanks, and cichlids.
Pros: natural look, gentle on barbels. Cons: compacts easily (anaerobic pockets), harder for deep-rooted plants.
Gravel
Medium grains, 2-5mm. Most forgiving beginner substrate. Allows water flow, won't compact.
Cons: food can fall between stones and rot. Not ideal for heavily rooted plants. Catfish fin damage possible.
Aqua Soil
Baked nutrient-rich clay (ADA, Fluval Stratum, Tropica). Lowers pH slightly, releases nutrients for 1-2 years.
Essential for high-tech planted tanks. Capping with sand for visual layers is popular in Iwagumi setups.
Layering Strategy
For heavily planted: 2cm aqua soil base + 3cm fine sand cap = nutrients + clean aesthetic.
Dirted tanks (Walstad): 2cm organic soil + 3cm sand cap. No more — anaerobic risk.