The Smell in a Sealed Tank Is Usually Ammonia, Not Mould

Open the lid and it hits you at nose level: sharp, slightly sweet, the smell of a cleaning product that has gone wrong. Keepers usually call that mould and go looking for white fuzz. Nine times out of ten there is no fuzz at all, and the smell is ammonia coming off a wet floor that has been accumulating waste for a month.

The two smells are easy to confuse at low concentration and completely different once you know them. Ammonia is sharp and catches at the back of the throat. Anaerobic decay is sour and rotten, like a pond dredged in summer. They have different causes and different fixes, and treating one as the other is how tanks get torn down for no reason.

Two Smells, Two Problems

Ammonia comes from nitrogen. Frog urine, urate, dead insects, rotting fruit and dead cleanup crew all break down into ammonium, which in a damp, warm, alkaline environment turns into ammonia gas. It accumulates because a sealed tank has nowhere to vent it and because the substrate is holding it in the water film rather than releasing it.

The sour smell is hydrogen sulfide and related compounds from anaerobic breakdown. It means the pore spaces have been full of water for too long and the aerobic community has died back. You will usually find it under the cap, not in the air, and it comes with grey or black patches and a slimy feel.

Why a Wet Floor Holds Ammonia

Water fills the pore spaces and slows gas exchange, so the ammonia produced in the substrate has to diffuse through a saturated medium before it reaches the air. That is slow. Meanwhile the same water keeps the organic matter breaking down. The result is a floor that holds a large nitrogen load in a small volume, ready to release it in a burst whenever you open the lid or disturb the surface.

This is also why the smell often seems to arrive suddenly. It did not arrive; it accumulated below the surface and the first good stir released it.

The Fix That Does Not Need a Teardown

Ventilation first, because it is free. Open 20 to 30 percent of the lid area, or run a small fan on the room rather than on the tank. Ammonia concentration in the air above the substrate drops quickly once there is exchange.

Then reduce the load. Pull the drainage water, replace 25 percent of the substrate, remove dead insects and old fruit, and stop feeding on the substrate itself. A feeding ledge keeps the waste out of the floor. Do not add activated carbon to the substrate; it adsorbs for a week and then becomes part of the problem.

• Test with an ammonia strip from an aquarium shop, diluted 1:10 in clean water, before you assume.
• One strip costs less than a bag of substrate and settles the argument.
• Fix the ventilation before you change the substrate, or the new floor will load up the same way.
• If the smell returns within a fortnight, the tank is overstocked or overfed, not under-cleaned.

The Counterintuitive Part

Tanks that are misted heavily and never allowed to dry tend to read high humidity and low air quality at the same time. The keeper sees 90 percent on the gauge and concludes the tank is healthy. The animal sits low, breathes shallow, and eats less, and the keeper adds more misting because the frog looks off.

Two of the worst-smelling tanks I have measured belonged to keepers with the cleanest routines: daily spot-cleaning, daily misting, no leaf litter. Wet, turned-over, oxygen-poor floors with fresh waste added every day. The tanks that smelled of soil were the ones with a litter layer, a cleanup crew and a wet-dry cycle.

Before you buy anything, open the lid, wait ten seconds, and decide which of the two smells you have. Then fix ventilation, not fragrance.