Where Plants Get CO2 Without Injection
In a no-CO2 tank, plants get carbon from: dissolved CO2 from atmospheric exchange (3–4 ppm), CO2 from fish and bacterial respiration (5–10 ppm in stocked tanks), and bicarbonates in hard water (limited).
Total: ~10 ppm CO2 in a stocked low-tech tank, vs 30 ppm in CO2-injected tanks. Plants grow about 1/3 the rate but most species still grow.
Lighting Balance
Lower light = lower CO2 demand = balanced tank.
Target: 25–40 PAR at substrate. Translates to: cheap LED fixtures, 6–8 hour photoperiod, no extra supplementation.
High light without CO2 = algae bloom guaranteed. The number-one mistake in no-CO2 tanks is buying lights that are too strong.
Liquid Carbon Supplements
Seachem Excel and similar products provide a CO2-like carbon source via glutaraldehyde. Dose 1ml per 10 gallons daily.
Excel also kills certain types of algae (BBA in particular) at higher doses. Caution: toxic to vallisneria, Anacharis, and some moss species.
Plant Selection for No-CO2
Always works: java fern, anubias, all crypts, vallisneria, Amazon sword, mosses, Bucephalandra, water wisteria.
Sometimes works: hygrophila stems, some ludwigias, some bacopas.
Almost never works: rotala stems, all carpet plants, demanding red plants.
Realistic Expectations
A no-CO2 tank looks acceptable at month 1, good at month 6, and beautiful at year 2. Patience is mandatory.
You will not achieve the dense, vibrant carpets shown in ADA catalogs. You will achieve a stable, low-maintenance tank that thrives for years.