Is Your Tank in Trouble?

Any ammonia above 0 ppm in a cycled tank is toxic. Fish will gasp at the surface, show inflamed red gills, and become lethargic. A 50% water change and a dose of Seachem Prime (which detoxifies ammonia for 24–48 hours) buys time. Identify the root cause: a dead fish, uneaten food, or a crashed filter are the most common culprits. Do not feed for 24 hours.

Nitrite blocks oxygen transport in fish blood — fish suffocate even in well-oxygenated water. Like ammonia, the safe level in a cycled tank is 0 ppm. Immediate 50% water change plus Seachem Prime. For freshwater tanks, 1 tablespoon of aquarium salt per 10 gallons helps: chloride ions compete with nitrite uptake at the gill membrane. Retest in 12 hours.

A freshwater pH below 6.0 is acutely stressful. Crashes happen in soft-water tanks with low KH (carbonate hardness) — KH is the buffer that holds pH stable. Never adjust pH more than 0.2 units per hour or you risk osmotic shock. Small water changes with harder tap water, crushed coral in the filter, or a seashell in the tank will raise KH and stabilise pH over hours.

Above 30°C oxygen dissolves poorly and most tropical fish experience heat stress. Turn off all lighting immediately, increase surface agitation (a fan across the water surface evaporates heat effectively), and float sealed bags of ice. Never pour ice directly into the tank — the temperature swing can be more dangerous than the heat itself. Target a drop of no more than 2°C per hour.

Nitrate above 40 ppm in freshwater (or 20 ppm in a reef) suppresses the immune system and stresses fish long-term. It rarely causes acute death but makes fish vulnerable to disease. A 25–30% water change with gravel vacuuming fixes the immediate problem. If your tap water itself is high in nitrate, mix it with reverse osmosis water, or add fast-growing stem plants like hornwort which consume nitrate directly.