Requirements for Red
High light (100+ PAR at tips). High CO2 (30-40 ppm stable). Iron-rich ferts (dose 0.5-1 ppm Fe weekly).
Nitrate should be moderate (5-15 ppm). Too high nitrate = green leaves instead of red.
Rotala Rotundifolia
Easiest red stem plant. Green in low light, red-pink in high light. Thick bushy growth.
Top-prune often to keep stems dense. New growth at top shows most intense red.
Alternanthera Reineckii
Deep burgundy red with white undersides. Mini version is smaller and denser. Cardinal is lighter red.
Needs stable CO2. Melts if CO2 crashes. Beautiful contrast against green plants.
Ludwigia Species
L. repens (rusty red), L. palustris (dark red), L. sp. Red (wine red). All need high light.
Ludwigia is more forgiving than Alternanthera — good gateway red plant.