Ammonia

Ammonia is the primary waste product produced by fish, uneaten food, and anything else organic that begins to decay in your tank. It’s also one of the most dangerous compounds in the aquarium environment, especially in new or uncycled tanks. Understanding how ammonia behaves in water—and what factors influence its toxicity—can help you spot problems early and respond effectively.

Ammonia vs. Ammonium

One of the first things to understand is that ammonia doesn’t exist in just one form. In water, it constantly shifts between two forms: uncharged ammonia (NH₃) and its charged counterpart, ammonium (NH₄⁺). The difference is subtle but incredibly important—ammonia is estimated to be around 200–300 times more toxic than ammonium. The key factor in this shift? pH.

At lower pH levels, ammonia tends to grab a hydrogen ion and becomes ammonium. Because ammonium carries a charge, it’s less able to pass through the gills of fish. In higher-pH systems, like most saltwater tanks, more of the total ammonia stays in the uncharged and more toxic form. This is why saltwater fish tend to be much more sensitive to ammonia than freshwater fish kept at lower pH.

Aquarium test kits don’t differentiate between the two forms—they report what's known as total ammonia nitrogen (TAN), which includes both ammonia and ammonium. So, a 0.5 ppm reading means something very different depending on your tank’s pH.

How Much Is Too Much?

In a healthy tank, ammonia should always be undetectable. If you’re seeing readings above 0.25 ppm, that’s a sign something’s off. At 0.5 ppm or higher, fish are likely in distress—even if you don’t see symptoms right away. Ammonia toxicity can cause a wide range of issues, from labored breathing to erratic swimming and even neurological symptoms. If left unchecked, it can quickly become lethal.

If you don’t have an established biofilter to handle ammonia, the fastest solution is a large water change. A 50% change or more can bring ammonia down to safer levels and buy you time to address the root of the issue.

The Good News: It’s Reversible

The effects of ammonia poisoning, while serious, aren’t necessarily permanent. If caught early and corrected quickly—either by a water change or transferring fish to clean, cycled water—most fish can recover fully. Timing and observation are key.

Preventing Ammonia Spikes

The best way to manage ammonia is to stop it from building up in the first place. This starts with having a mature, cycled filter or seeding a new one with established media to give your beneficial bacteria a head start. Strong filtration and regular maintenance go a long way toward keeping levels in check.

Be mindful of anything that breaks down in the tank. Uneaten food, dead plant matter, or deceased fish are all protein sources that degrade into ammonia. Overfeeding is a common mistake—what doesn’t get eaten becomes a liability. When in doubt, feed less and remove excess. It’s better to underfeed than to risk a spike that could crash your system.

Want to know how much of that ammonia is actually dangerous?

You can estimate how much of your total ammonia is in the toxic NH₃ form by using your tank’s pH and temperature. As a general rule, the higher the pH and the warmer the water, the more toxic ammonia you’ll have. There are online calculators and charts (like this one from Hamza’s Reef) that can do the math for you. It’s a handy way to put those test results in context, especially in high-pH systems like saltwater tanks. Playing around with different numbers gives you an idea of just how toxic ammonia can be. When pH raises by 1, the ammonia to ammonium ratio shifts by roughly 10x.