Glutaraldehyde in Aquariums: Liquid Carbon and Algae Control
Walk into any planted tank discussion and someone will mention “liquid carbon” — a product category built largely around glutaraldehyde, a chemical compound that functions as both an algaecide and a marginal carbon source for aquatic plants. This glutaraldehyde aquarium dosing guide from Gensou Aquascaping in Singapore cuts through the marketing to explain what glutaraldehyde actually does, how to dose it safely, and where it fits alongside pressurised CO2. Used correctly, it is a powerful tool; used carelessly, it can harm livestock and sensitive plants.
What Is Glutaraldehyde?
Glutaraldehyde is an organic compound (C5H8O2) used widely as a disinfectant and fixative in medical and industrial settings. In the aquarium hobby, it appears at low concentrations — typically 1.5–2.5% — in products like Seachem Excel, APT Fix, and various generic “liquid carbon” formulations. At these diluted concentrations, it provides a mild carbon supplement through a pathway plants can partially utilise, while simultaneously killing algae through protein cross-linking. It is not a direct replacement for CO2 gas, but it serves a useful niche.
How It Works in Planted Tanks
Plants can utilise the carbon released as glutaraldehyde breaks down in water, though the efficiency is far below that of dissolved CO2 from pressurised systems. The real value lies in its algaecidal properties. Glutaraldehyde disrupts cellular proteins in algae, particularly effective against black brush algae (BBA), staghorn algae, and hair algae. At standard daily doses, it suppresses new algae growth. At spot-treatment concentrations applied directly via syringe, it kills existing colonies within days, turning them pink or white before they disintegrate.
Daily Dosing Protocol
For general supplementation and algae prevention, dose 1 ml per 20 litres daily. Add the dose in the morning, ideally at lights-on when plants begin photosynthesising. Use a syringe or measuring pipette for accuracy — eyeballing doses with a bottle cap is unreliable and risks overdosing in smaller tanks. Glutaraldehyde degrades within 24 hours in aquarium water, so daily dosing maintains a consistent level. Start at half the recommended dose for the first week to observe livestock response before increasing to full strength.
Spot Treatment for Algae
Direct application is the most effective use of glutaraldehyde against established algae. Turn off all flow and filtration. Draw undiluted product into a syringe and apply directly onto algae-covered surfaces — driftwood, rocks, plant leaves — holding the syringe tip 1–2 cm from the target. Leave flow off for 5–10 minutes to allow contact time. Restart filtration and perform a 20% water change the following day. Treated BBA turns pink within 48 hours and is then consumed by amano shrimp and nerite snails. Limit spot treatments to twice per week.
Safety Precautions
Overdosing glutaraldehyde harms fish, shrimp, and sensitive plants. Symptoms of overdose include fish gasping at the surface, shrimp becoming lethargic or turning opaque, and delicate plants like Vallisneria and certain mosses melting. Never exceed 1.5 ml per 20 litres as a daily dose. Some species are particularly sensitive: Vallisneria species, riccia, and certain Rotala variants may decline even at standard doses. If you keep these plants, start at quarter-dose and monitor closely over two weeks. Always store the product away from children and pets — concentrated glutaraldehyde is a skin and eye irritant.
Glutaraldehyde vs Pressurised CO2
Pressurised CO2 is categorically superior as a carbon source. It dissolves directly into water, is used efficiently by plants through normal photosynthesis, and supports growth rates that glutaraldehyde cannot approach. Where glutaraldehyde holds an advantage is in simplicity and cost: no cylinder, no regulator, no diffuser — just a bottle and a syringe. For low-tech tanks without CO2 injection, daily glutaraldehyde dosing provides a measurable growth boost. In high-tech tanks already running CO2, glutaraldehyde’s main role shifts to targeted algae control rather than carbon supplementation.
Product Availability in Singapore
Seachem Excel is the most widely stocked branded option, available at most aquarium shops and on Shopee for around $15–$25 per 250 ml bottle. Generic 2% glutaraldehyde solutions from local sellers on Carousell cost significantly less — often $5–$8 for 500 ml — and are chemically identical for aquarium purposes. APT Fix from 2Hr Aquarist, a Singapore-based company, is another popular choice with a loyal local following. Whichever product you choose, check the concentration and adjust dosing accordingly.
When to Use and When to Skip
Glutaraldehyde makes sense for low-tech planted tanks seeking a growth boost, as a targeted algae weapon in any setup, and as insurance during CO2 system downtime. Skip it if you keep Vallisneria-heavy tanks, sensitive shrimp breeding colonies where any chemical risk is unacceptable, or if your tank already has stable, algae-free conditions with pressurised CO2. This glutaraldehyde aquarium dosing guide should help you decide where this tool fits in your planted tank maintenance routine.
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