Whether your aquarium plants need CO2 depends on light intensity + plant species. No-CO2 tanks (low light): anubias, java fern, java moss, cryptocoryne, amazon sword, vallisneria, hornwort, water sprite, dwarf sagittaria. Run light at 4,000-6,500K, 6-7 hours, no fertilizer or 1x weekly Flourish Comprehensive. Liquid carbon tanks (medium light): rotala rotundifolia, ludwigia repens, cabomba, bacopa, alternanthera. Dose Seachem Excel or Tropica Premium. Caveat: Excel kills vallisneria and damages anacharis. Pressurized CO2 tanks (high light, high tech): required for HC cuba, dwarf hairgrass carpets, monte carlo, glossostigma, riccia, blyxa, most stem plant carpets, AR mini, all reds. Target 25-30 ppm CO2 (use a drop checker + KH/pH math). Inject during photoperiod only. Cost: liquid carbon $15/month, full pressurized CO2 setup $250-450 one-time + $20/refill every 4-6 months. Plant nutrition stack: light + CO2 + macros (NPK) + micros (trace) + substrate. Skip any one and the others can't compensate.
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