Substrate: The Engine Room

Substrate is not just "flooring" for your tank. In a planted aquarium, it is the primary source of nutrition for root feeders.

CEC (Cation Exchange Capacity)

This is the most important scientific concept for substrates. CEC measures the substrate's ability to hold onto positively charged nutrient ions (Cations) like Calcium, Magnesium, Potassium, and Iron.

• High CEC (Aqua Soil, Clay): acts like a battery. It pulls nutrients from the water column, stores them, and releases them to plant roots on demand.
• Low CEC (Sand, Gravel): Inert. It holds nothing. Nutrients wash right past it.

Types of Substrate

1. Active Soils (High CEC)

• Pros: Lowers pH, rich in ammonia/nitrogen, promotes massive root growth.
• Cons: Expensive, can cloud water if disturbed, eventually depletes (1-2 years).

2. Inert Sand (Zero CEC)

• Pros: Cheap, easy to clean, everlasting.
• Cons: No nutrients. You MUST use Root Tabs to feed plants.

3. Walstad / Dirted

• Pros: Natural soil capped with sand. Extremely nutrient-rich.
• Cons: Messy if you pull up plants. Risk of anaerobic pockets.

Anaerobic Pockets

If a substrate is too deep and compact (like fine sand > 3 inches), oxygen cannot penetrate. Bacteria that don't need oxygen begin to thrive. They produce Hydrogen Sulfide ($H_2S$) gas.

• Sign: Black spots in the sand, rotten egg smell.
• Danger: Highly toxic to fish roots and fish if released.
• Solution: Poke the substrate occasionally or use Malaysian Trumpet Snails to aerate it.