Planted tank basics: substrate, lighting, and CO2

Choose a substrate for the plants you actually want

Root-feeding plants — many Cryptocoryne, sword plants, and most stem plants — benefit from a nutrient-rich substrate or root tabs pushed into an inert substrate. Plants that take nutrients mainly through their leaves, including many mosses and epiphytes tied to wood or rock, are far less dependent on what is in the substrate itself.

A dedicated planted substrate can supply nutrients for months before it needs supplementing, while plain gravel or sand paired with root tabs is a lower-cost way to support root feeders. Cap a nutrient substrate with a thin inert layer if the product instructions recommend it, and rinse inert substrates thoroughly before use to avoid cloudiness.

Light is intensity and duration together

Photosynthesis in aquatic plants depends on available light energy and available carbon together, and pushing one up without the other creates an imbalance the tank has to absorb somehow. A fixture's wattage alone says little about what actually reaches the plants; usable light output, spectrum, and how far it travels through the water column all matter more.

Start conservatively: moderate intensity for six to eight hours, rather than maximum output for a long day. Increase gradually and watch plant growth and algae response before increasing further. A photo period that is too long relative to available nutrients and carbon is one of the more common routes into an algae problem, because algae exploits exactly the imbalance plants cannot yet use.

Carbon dioxide: why some tanks need it and some don't

Aquatic plants use dissolved carbon dioxide, along with light and nutrients, to build new tissue through photosynthesis. In a lightly planted tank under moderate light, the CO2 that occurs naturally from fish and bacterial respiration, plus surface gas exchange, may be enough. In a densely planted tank running higher light, demand for carbon can outpace what the water already contains, and growth stalls even though light and nutrients are otherwise adequate.

This is the practical dividing line between a "low-tech" planted tank — undemanding plants, moderate light, no injected CO2 — and a "high-tech" one that injects CO2 to support faster-growing, higher-light species. Neither approach is more correct; they suit different plant selections, budgets, and maintenance appetites.

If you inject CO2, protect the fish first

Dissolved oxygen and dissolved CO2 move in opposite directions across the day in a CO2-injected tank: CO2 is highest and oxygen often lowest toward the end of the light period, then the reverse overnight as plants stop consuming CO2 and start respiring like the rest of the tank. Fish sit at the bottom of that oxygen curve.

Follow the equipment manufacturer's instructions for injection rate and timing, use a drop checker or equivalent monitoring method rather than judging by bubble count alone, and increase surface agitation or pause injection overnight if the system does not already do so automatically. Introduce CO2 gradually on a new setup and watch fish behavior — labored breathing or gasping at the surface is a signal to reduce injection and increase surface gas exchange immediately, not to wait and see.

Feed the plants, not just the fish

Beyond carbon and light, plants need macro- and micronutrients including nitrogen, phosphorus, potassium, iron, and other trace elements. A moderately stocked, moderately fed tank supplies some of this from fish waste, but a heavily planted or high-light tank commonly needs supplemental fertilization to avoid a nutrient becoming the limiting factor — which, again, tends to show up as an algae problem rather than as an obvious plant symptom at first.

Introduce a liquid or substrate-based fertilizer at the product's starting dose, watch plant growth and leaf color over a couple of weeks, and adjust from there. Large sudden increases in any single input — light, CO2, or nutrients — are more likely to trigger algae than a steady, moderate baseline.

Start simple, add complexity deliberately

A first planted tank does better with a smaller number of undemanding, low-light species — many Anubias, java fern, various Cryptocoryne, and mosses tied to hardscape — than with an ambitious high-tech layout on day one. Undemanding plants tolerate the inevitable early mistakes in light, dosing, and CO2 timing far better than fast-growing stem plants do, and a stable low-tech tank is a better foundation to build from than an unstable high-tech one.

Sources and further reading

Each source is listed with the part of this guide it supports. Source scope matters: a taxonomy record, product label, or general husbandry page does not automatically support every care claim.

• University of Florida IFAS: Dissolved Oxygen — Plant Management in Florida Waters
  Aquatic-plant photosynthesis, carbon dioxide use, and the oxygen this process releases into the water.
• University of Florida IFAS: Ammonia in Aquatic Systems
  How pH and alkalinity interact with dissolved gases and nutrient chemistry in a closed aquatic system.
• Merck Veterinary Manual: Providing a Home for Fish
  Substrate, lighting, filtration, and general environmental setup considerations for an aquarium.