Skip to Content

What Is The Best pH Level For Molly Fish?

Disclosure: When you purchase something through my affiliate links, I earn a small commission. As an Amazon Associate, I earn from qualifying purchases.

Molly fish are relatively easy to care for, making them one of my favorite types of freshwater aquarium fish.

However, maintaining their ideal pH can sometimes be challenging. What is the ideal pH for molly fish? What can cause it to lower or rise? And how can you properly adjust it?

These were the very questions I had when I first started this hobby. In this article, I will provide detailed answers to each of them, leaving you with no lingering uncertainties.

So, let’s delve into it.

The Best pH For Molly Fish

Molly fish thrive best in water with a pH range of 7.5 to 8.5.

This slightly alkaline condition is optimal for their health, growth, and reproduction. Understanding the specifics of this can ensure your molly fish thrive.

Consider the following:

  • Ideal pH Range: Molly fish naturally inhabit freshwater and brackish environments in Central and South America, where the water typically has a slightly alkaline nature. This factor makes them distinct from fish species that prefer more acidic conditions.
  • Health and Well-being: If the water’s pH level deviates from the ideal range, molly fish can suffer from increased stress and susceptibility to disease. This also affects their color vibrancy, which is a significant aspect of their overall health and vitality.
  • Reproductive Success: Molly fish, as livebearers, find a suitable pH level crucial for successful breeding. A well-regulated pH environment encourages healthier offspring and boosts their overall population in your aquarium.
  • Importance of pH Stability: Sudden or drastic changes in pH levels can lead to shock in molly fish, severely affecting their well-being. Regular monitoring and gentle adjustment of water pH can help maintain the stability that mollies need.
  • Role of Water Hardness: Molly fish thrive in hard water, which typically aligns with a higher pH level. Hardness levels between 10-25 dGH not only support the right pH but also provide essential minerals, contributing to their health and growth.

Also Read: Molly Fish Tank Setup

Can Low pH Affect Molly Fish?

Yes, low pH can indeed negatively affect molly fish.

When the pH levels drop below 7.5, it induces considerable stress in molly fish, potentially resulting in health issues and reduced vitality.

Let’s further elaborate on how low pH can impact the life and health of a molly fish.

  • Health Deterioration: When water’s pH falls below the ideal range, molly fish can experience health issues. These can include lethargy, loss of color, and increased susceptibility to diseases such as fin rot or fungal infections.
  • Stress Induction: Molly fish, when subjected to low pH levels, can exhibit signs of stress. This can manifest in erratic swimming, loss of appetite, or constant hiding, which significantly impact their quality of life.
  • Breeding Difficulties: Reproduction in molly fish can be adversely affected by low pH levels. Eggs may not be fertilized effectively and the overall reproductive health of the fish may be compromised, resulting in fewer offspring.
  • Reduced Lifespan: Persistent exposure to low pH can shorten the lifespan of molly fish. The continual stress and heightened disease susceptibility can lead to premature death.
  • Metabolic Disruptions: Low pH can disrupt the metabolic processes in molly fish. This can result in abnormal growth rates and a weakened immune system, making it harder for them to fend off diseases.

How To Determine The pH Level In Your Aquarium

You can measure the pH level in your aquarium by using a dedicated testing kit:

  • Acquire a pH test kit: Choose a kit suited to freshwater environments. Make sure it’s designed for a broad pH range to cater to the needs of molly fish. My recommendation: API FRESHWATER MASTER TEST (link to Amazon).
  • Sample the aquarium water: Collect water using a clean container, ensuring it’s taken from a mid-level depth. This approach helps avoid disturbing the substrate, which could impact your molly fish.
  • Run the pH test: Follow the kit instructions meticulously. This will help achieve an accurate reading and maintain a suitable environment for molly fish.
  • Analyze the results: Match the resulting color to the provided chart. Use the comparison to get an accurate pH reading relevant to your molly fish.
  • Adjust pH if necessary: If the pH is out of the desired range, use pH modifiers. Keeping the water in the optimal pH range is crucial for the health of your molly fish.

How To Increase The pH Level In Your Aquarium

To increase the pH level in your aquarium, particularly for species like the molly fish, one of the most effective methods is using commercial pH adjusters or buffers.

These are readily available products specifically designed to safely alter the pH level in your aquarium.

However, the process requires careful implementation to avoid abrupt changes that could stress your fish. Here are specific steps you can follow:

  • Select a Suitable pH Modifier: Opt for a commercial product that increases pH levels. Make sure it is safe for the type of aquarium and fish you have, especially molly fish. My recommendation: API PROPER pH 8.2 (link to Amazon).
  • Follow the Product Instructions: Each pH modifier will have instructions regarding dosage based on your aquarium size. Be sure to follow these closely to create a safe environment for your molly fish.
  • Make Adjustments Gradually: Introduce the pH modifier slowly to prevent drastic changes. Rapid shifts in pH can cause stress or even harm to your molly fish.
  • Monitor pH Levels Frequently: Regularly test the pH in your aquarium using your pH test kit. This allows you to track changes and ensures the environment remains suitable for your molly fish.
  • Maintain Consistency: Once the desired pH level is achieved, continue regular monitoring and adjustments as needed. Consistent pH levels contribute to a healthier environment for your molly fish.

How To Decrease The pH Level In Your Aquarium

Decreasing the pH level in your aquarium can be achieved by using commercial pH reducers, incorporating naturally acidic substances such as driftwood, or increasing the carbon dioxide levels in your tank.

Care must be taken to gradually introduce these changes to avoid shocking your molly fish with abrupt pH shifts:

  • Select a Suitable pH Reducer: Opt for a commercial pH reducer designed for aquariums. Ensure it’s suitable for your aquarium type and safe for molly fish. My recommendation: API Proper pH 7.5 (link to Amazon).
  • Follow Product Instructions: Adhere to the dosing instructions on the product, which are typically based on the size of your aquarium. Accurate dosing is essential to maintain a healthy environment for molly fish.
  • Introduce Changes Gradually: Adjust the pH slowly to avoid rapid changes, which can stress or harm your molly fish.
  • Regularly Monitor pH Levels: Use your pH test kit to frequently check the aquarium’s pH. Regular monitoring helps maintain optimal conditions for your molly fish.
  • Maintain Consistent pH Levels: Once the desired pH level is reached, ensure regular monitoring and adjustments to keep the pH steady. Consistent conditions contribute to a healthier habitat for your molly fish.

What Leads To A Drop In pH Levels?

If the pH in your molly fish tank is too low, it is probably due to one of the following:

1. Biological Processes

Aquarium inhabitants like molly fish and their biological processes contribute to the decrease in pH.

Fish waste, decaying food, and plant material produce nitrogenous wastes, which eventually turn into nitric acid, lowering the pH:

  • Fish Waste: Fish, including molly fish, excrete waste into the water, which can lead to the formation of nitric acid, thereby decreasing pH levels.
  • Decaying Organic Matter: Decaying plants and leftover food in the aquarium also contribute to nitric acid formation, causing a drop in pH.
  • Biological Filtration: The nitrification process carried out by beneficial bacteria in your aquarium’s filter converts ammonia to nitrate, producing acid and reducing pH.

2. Aquarium Substrate

Specific types of tank substrates, like peat moss or certain driftwood varieties, have the potential to lower the pH of your aquarium water.

This decrease in pH can impact the well-being of your molly fish:

  • Peat Moss: If used as a substrate, peat moss can leach acids into the water, lowering the pH levels over time.
  • Driftwood: Certain types of driftwood release tannins, which are acidic and will decrease the water’s pH.
  • Unwashed Substrates: If not properly rinsed before being added, certain substrates can carry residues that might affect the pH.

3. Tannins

Tannins, often released from driftwood or certain types of leaves, can naturally decrease the pH level in your aquarium, potentially impacting molly fish:

  • Driftwood: Certain driftwoods release tannins when submerged, these organic compounds are acidic and can lower the pH.
  • Aquatic Plants: Some aquatic plants or the addition of leaves like Indian almond leaves can release tannins, decreasing the pH levels.
  • Overstocking Plants: Overcrowding your aquarium with plants can lead to excess organic matter, which when decayed, produces tannins.

4. Low-pH Tap Water

The pH of your source water can also affect the pH of your aquarium.

If your tap water is naturally acidic, it could lower the pH in your aquarium, potentially creating unfavorable conditions for your molly fish:

  • Tap Water pH: The pH of tap water can naturally be lower, depending on its local characteristics. This can cause a decrease in the aquarium’s pH when used for water changes.
  • Rain Water: If you use collected rainwater, it may have a lower pH due to dissolved carbon dioxide, which can affect the aquarium’s pH.
  • Water Purifiers: Certain water purifiers or softeners possess the capability to decrease the pH of your tap water.

What Leads To An Increase In pH Levels?

On the other hand, if the pH in your molly fish is too high, it is probably because of one of the following:

1. High Carbonate Hardness (KH)

High carbonate hardness in your aquarium can elevate the pH.

This occurs because carbonates in the water act as a buffer, increasing pH, which can affect molly fish if the levels get too high.

Let’s delve into this concept:

  • Natural Carbonates: If your water source contains high carbonates, it can elevate the pH, potentially making conditions unsuitable for molly fish.
  • KH Additives: Certain additives designed to increase KH can also raise your aquarium’s pH levels, which can affect the health of molly fish.
  • Certain Substrates: Some substrates, such as crushed coral or aragonite, intended for marine tanks, can increase both KH and pH.

2. Insufficient Carbon Dioxide (CO2)

Insufficient carbon dioxide in the aquarium can lead to an increase in pH.

CO2 helps to balance pH, so lower levels can cause pH to rise, affecting the comfortable environment for your molly fish.

Let’s explore this topic:

  • Inadequate CO2 Supply: If your planted aquarium isn’t supplied with adequate CO2, the pH may rise, impacting the molly fish’s environment.
  • Daytime pH Rise: During the day, plants photosynthesize and consume CO2, which can raise pH, affecting your molly fish.
  • Improper Gas Exchange: If your aquarium doesn’t have sufficient surface agitation or aeration, this could limit CO2 exchange and potentially increase pH.

3. Excessive Alkaline Substances

The addition of alkaline substances can raise the pH level in your aquarium. Overuse can lead to drastic pH changes that can be harmful to molly fish.

Now, let’s discuss this in more detail:

  • Alkaline Additives: Certain pH buffers or commercial products can raise pH levels when used excessively, which can harm molly fish.
  • Decorative Elements: Some aquarium decorations or substrates like limestone and crushed coral can release alkaline minerals, elevating pH levels.
  • Improper Dosage: Using too much of a water conditioner or buffer could spike the pH, creating an unsuitable environment for molly fish.

4. Inadequate Filtration Or Water Changes

Inadequate filtration or infrequent water changes can also result in increased pH.

Filtration helps remove substances that can raise pH, and water changes help maintain a stable pH, both crucial for the wellbeing of molly fish.

Here are some points to consider:

  • Insufficient Filtration: Inadequate filtration can allow pH-increasing substances to accumulate, affecting the comfortable habitat for your molly fish.
  • Infrequent Water Changes: Neglecting regular water changes can result in a gradual increase in pH, which may not be ideal for molly fish.
  • Unclean Filters: If filters aren’t cleaned regularly, accumulated waste can break down and release substances that raise pH.

Additional Water Requirements For Molly Fish

Besides the water pH, there are two other important parameters when it comes to molly fish, being water hardness and temperature.

1. Water Hardness

Molly fish prefer hard, alkaline water conditions.

Ideally, the water hardness for molly fish should be between 10-25 dGH (degrees of General Hardness) and the carbonate hardness (KH) should be 7-12 degrees.

2. Temperature

Molly fish prefer warm tropical conditions, thriving in water temperatures of 72-78°F (22-26°C).

Maintaining this temperature range mimics their natural habitat and supports their overall health.

Also Read: Molly Fish Temperature


If you are in a rush, here is a brief summary of what I discussed earlier:

  • Molly fish prefer water with a pH range of 7.5 to 8.5, which is similar to their natural habitat and helps them stay healthy and thrive.
  • Low pH levels can cause increased stress, susceptibility to diseases, and decreased color vibrancy in molly fish, compromising their overall health and vitality.
  • Maintaining a stable pH environment is crucial for the successful breeding of molly fish and the production of healthier offspring.
  • Sudden or drastic changes in pH can lead to stress and harm to molly fish, highlighting the importance of monitoring and maintaining pH stability in the aquarium.
  • Factors such as biological processes, substrate choice, the presence of tannins, and tap water characteristics can contribute to fluctuations in pH levels in the aquarium.
  • Regular monitoring and proper adjustments are necessary to ensure the pH remains within the optimal range for the well-being of molly fish in the aquarium.