Planted Freshwater Aquarium Chemistry

If you're new to the fishkeeping hobby, the chemistry of a planted freshwater aquarium can look extremely intimidating and complicated. What's the difference between nitrate and nitrite? How did ammonia get into your tank? What's the deal with this dissolved oxygen stuff?

The water chemistry of a planted aquarium is actually far simpler, and far less daunting, than it may seem. Here's a basic overview of the chemicals found in a planted freshwater aquarium.

Nitrite

Nitrite is a major discussion-point for enthusiasts of freshwater aquarium chemistry. Nitrite is an important nutrient, but it is extremely toxic to almost all species of freshwater fish. When organic matter, such as dead leaves or fish poop, decays, it releases nitrite into the water. In a new aquarium, this scary chemical lingers and can quickly cause the death of every fish in the aquarium. Fortunately, if your aquarium is well-established, it turns into nitrate. Your nitrite levels should be undetectable after your aquarium is established.

Nitrate

Despite the similarity in name and chemistry, nitrate and nitrite are functionally quite different. Specialized bacteria found in your filter, water and substrate consume nitrite and convert it into nitrate. In a planted freshwater aquarium, your benthic plants will absorb much of the nitrate in your water. If it is left unchecked, nitrate levels can cause algae blooms and fish death. Nitrate levels should ideally stay below 20 parts per million in an established planted aquarium, but most fish can tolerate levels as high as 60 parts per million for short periods of time. Note that nitrate supports plant growth, despite its reputation as a fish-killer.

Ammonia

When you hear the word ammonia, "poison" should be the first word that pops into your head. Ammonia from fish poop, fish food and decaying plant matter can build up in your aquarium's chemistry, and even relatively low levels can cause your fish to die-- quickly. If you notice an unpleasant or urine-like odor emanating from your water, get to work immediately on doing a large water change. Aquatic plants can help to slightly reduce ammonia levels, but it ultimately takes deligent owner-involvement to keep ammonia levels where they need to be.

Phosphate

Most people who keep planted freshwater aquariums do not need to test for phosphate. This nutrient is not particularly toxic to fish, but it might be worth checking for if you experience algae blooms. Phosphate from fish food and polluted tapwater can help to encourage the growth of your live aquatic plants, and your benthic plants are instrumental in keeping levels low. However, for the most part, there's really no need to be concerned about your aquarium's phosphate levels.