Can You Use Mollies to Cycle a Marine Tank?

Have you seen people breed mollies in a marine tank before? Do you think mollies can thrive in a marine tank?

In this article I will cover everything you need to know about mollies and how they contribute to cycling a marine tank.

Can You Use Mollies to Cycle a Marine Tank? Yes, mollies can be used to cycle a marine tank but they are not the best fish to use because they can be sensitive to the ammonia and nitrite levels in your tank.

Can You Use Mollies to Cycle a Marine Tank

What Does it Mean to Cycle a Marine Tank?

Cycling a marine tank involves introducing nitrogen cycling to your saltwater tank to make it favorable for saltwater fishes to thrive; the process takes about 30 days to complete.

Nitrogen cycling in saltwater converts ammonia and nitrite— little concentration of these compounds is very harmful to fishes— present in the water to nitrate. Nitrates are less toxic to fishes and can be controlled in the tank by regular changing of water.

Are Stallions Allowed To Race

This must-do cycle in every new tank ensures that the environment is healthy and safe for the fish to live, as lack of cycling can lead to fish death.

Introducing the nitrogen cycle in a marine tank can be done using ammonia, high concentration bacterial compounds that digest aquarium wastes and introduces biological filtrates into the tank, live rock, sand, fish feed, or fishes.

YouTube video

How do Mollies Cycle a Marine Tank?

Generally, mollies naturally thrive in salty water and are hardy (they can survive in an area with high toxins). Hence, they can use it to cycle saltwater tanks, especially the Black mollies.

When using black Mollies to cycle your SW tank, introduce it along with some plants, marine salts, beneficial bacterias like Nitrosomonas, and nitrifying bacterias into the tank; the number of fishes you present depends on the concentration of nitrate.

When the fish or fishes are introduced into the tank excrete, their waste containing ammonia helps the bacterias in the tank bloom.

The nitrifying bacterias in the tank convert the ammonia to nitrite, while the other beneficial bacterias convert the nitrite to nitrate.

As the cycling process is ongoing, replace about 25 percent of the water in the tank every few days to help keep the toxin level of the water minimal.

During the cycling period, use a test kit to monitor the toxicity level of the water by checking the levels of ammonia, nitrite, and nitrate daily until it gets to 0mg/l. The moment the ammonia and nitrate level get to 0mg/l, your cycling is complete, and the tank is safe for saltwater fishes.

Should Mollies be Used to Cycle a Tank?

Here are reasons why you should use mollies to cycle a tank.

1. Readily Available

You do not need to stress yourself looking for mollies as they can be found in literally any fish store. Look for fish stores within or around your locality to buy these cute fishes. If you have freshwater ponds close to you, you can go there to source mollies.

2. Affordable

Mollies are very cheap to buy compared to other saltwater fishes. You can get different colors of mollies, including the black-colored ones, for as low as $2. The price ranges from $2 to $8, depending on the fish store.

3. Hardy

Mollies have high resistance to extreme conditions. They can survive in saltwater which makes them the best choice for cycling marine tanks and freshwater tanks. Instead of dying due to the toxicity of the water, they adjust and find ways to thrive.

4. Active

Mollies are cute and hardy, but they are also very active fishes, and that is why most people like to introduce them into their tanks. They move around the tanks trying to attach themselves to the walls of the tanks, causing the environment in the water to become lively.

5. Cleanup

Saltwater tanks are very prone to algae. However, mollies love to feed on algae, making them good cleaning agents for your tank asides from cycling agents. As they move around the tanks, they eat most undesired components, keeping them clean.

6. Safe

Most salt fishes are usually quarantined before introducing them into the marine tank to avoid introducing diseases and pests that are not healthy for the tank. However, mollies are freshwater fishes and will not need to be quarantined; you can put them directly into the tank. Hence, they save you the time and stress of quarantining the fish.

7. Dither Fish function

Since the tank is new, the other fishes might feel reluctant to move around, but with the active movement of mollies, the fishes get motivated and assured that the environment is safe for them. Hence, mollies boost the activity and activeness of other fishes in the tank.


Asides from serving aesthetic purposes, we can use mollies to cycle marine water fishes.

They are affordable, hardy, readily available, and essential in maintaining the chemical balance of a saltwater tank.

Their feed of mollies is easy to get, and they are easy to rear. However, most people prefer using ammonia to fishes to carry out their marine tank’s cycling process for personal reasons.



Hello, I'm Jason. I'm the guy behind I volunteer at my local fish shop and I created this site to offer tips and advice on the fish I care for.

Recent Posts