How do I respond to ammonia in a marine aquarium?

Why this happens

Ammonia becomes more toxic as pH and temperature rise. Marine systems commonly operate at higher pH than freshwater, so measurable un-ionised ammonia can be especially dangerous.

• A dead animal may be hidden in rockwork.
• A new or damaged biofilter may not process the load.
• Medication or cleaning can reduce microbial activity.
• Test kits may read total ammonia differently after conditioners.

Reef organisms can be affected before fish show obvious signs. Check snails, shrimp, corals and smell, and inspect overflows and filter chambers for decay.

What to check first

• Are fish breathing rapidly or hiding?
• Are corals closed and snails inactive?
• Did salinity, temperature or pH change?
• Has a fish, anemone or invertebrate disappeared?

Compare the result with the aquarium’s normal baseline, not only a generic target. Record the time, recent maintenance, new livestock, feeding changes, source-water details and any equipment fault. A short timeline often separates the original cause from secondary symptoms.

A practical action plan

1. 1. Increase oxygen and stop feedingReduce demand while the source is found.
2. 2. Remove decayInspect rock, sump, overflow and filter socks.
3. 3. Change water safelyUse temperature- and salinity-matched new saltwater.
4. 4. Protect and monitor filtrationRetain mature media and test repeatedly for several days.

How to interpret the response

Falling ammonia after source removal and changes is encouraging, but a rebound shows the biofilter or hidden source is still unresolved.

Change one important variable at a time where welfare allows, then observe and retest. When several products or adjustments are made together, it becomes difficult to know what helped, what caused stress and whether the underlying problem remains.

What to avoid

Do not add dry salt, force pH upward, replace all media, dose several ammonia products or rely on a conditioner without retesting.

When it is urgent

Measurable ammonia with gasping fish, widespread coral slime, anemone decay or loss of balance is an emergency.

How to prevent it next time

Quarantine additions, secure livestock, inspect hidden chambers and keep emergency mixed saltwater and aeration available.

Keep enough records to recognise the aquarium’s normal behaviour, water chemistry and equipment performance. Trends are more useful than isolated numbers, and early correction is usually safer than a large emergency change.