Complete Guide : Fish for 40 gallon tank


Envision having a piece of the ocean in your living room, swarming with life and colour. A 40-gallon tank is the palette for your underwater masterpiece, a sweet spot that stabilities the freedom to experiment with diverse aquatic life and the handy maintenance it offers. Whether you’re a greenhorn embarking on your first fish-keeping journey or an experienced aquarist looking for new insights, this comprehensive guide will provide you with invaluable guidance on selecting the perfect fish for your 40-gallon tank.

Understanding Tank Capacity and Fish Size

Before we embark on our journey to discover the ideal fish for your 40-gallon tank, it’s important to grasp the nuances of tank capacity and fish size. However many aquarists are familiar with the “inch per gallon” guideline, so it’s worth delving deeper into its ramifications.

The “Inch per Gallon” Rule

The “inch per gallon” rule serves as a central guideline in the world of fishkeeping. It implies that for every gallon of water in your tank, you can accommodate one inch of fish. While this rule can be a useful starting point for small tanks, it becomes progressively less accurate as tank size increases.

The Importance of Adult Size

In the context of a 40-gallon tank, one must prioritize considering the adult size of the chosen fish species. Some fish may seem appropriate when they are young, but they can hastily overrun your tank as they mature. Overstocking can lead to a host of issues, including compromised water quality, stressed fish, and increased susceptibility to diseases.

Best Fish Options for a 40-gallon Tanks

Let’s dive deeper into the practical side of fish selection. Here are five fascinating fish species that are well-suited for your 40-gallon tank:

1. Angelfish:

  • Appearance: With their tall, triangular bodies and long, graceful fins available in various patterns and colours, Angelfish are charming to behold.
  • Temperament: Angelfish fall into the semi-aggressive category, but they can cohabit with non-fin-nipping fish.
  • Diet: These omnivores enjoy a varied diet of flake food, live food, and pellets.
  • Care Level: Considered intermediate because of their precise care needs.
  • Insight: Be mindful that Angelfish can grow quite tall and are territorial, so provide them with ample space.

2. Clown Loaches:

  • Appearance: Clown Loaches sport luminous orange bodies decked with distinctive black stripes.
  • Temperament: These fish are known for their calm but active behaviour.
  • Diet: As omnivores, they thrive on sinking pellets, worms, and the occasional helping of veggies.
  • Care Level: Minimal assistance is needed to keep Clown Loaches happy and healthy.
  • Insight: Clown Loaches are gregarious creatures, so consider keeping them in groups to ensure their well-being.

3. Pearl Gourami:

  • Appearance: Pearl Gouramis boast a silvery-blue body tittivated with a distinctive pearl-like pattern.
  • Temperament: Generally serene and get along well with other fish species.
  • Diet: These omnivores enjoy a diet that includes flake food, live food, and vegetables.
  • Care Level: Moderate level of care is required.
  • Insight: Pearl Gouramis thrive in environments with dense vegetation, so be sure to provide them with suitable plant cover.

4. Rainbowfish:

  • Appearance: Characterized by their slender bodies and mesmerizing iridescent colours, Rainbowfish are a visual delight.
  • Temperament: They are active swimmers and are generally peaceful.
  • Diet: Rainbowfish are omnivores and relish a diet of flake food, live, and frozen food.
  • Care Level: They are relatively easy to care for, making them suitable for aquarists of various experience levels.
  • Insight: Rainbowfish are at their best when kept in schools, so consider having a group of these vibrant swimmers.

5. Kribensis (African Cichlid):

  • Appearance: Kribensis are known for their striking and vibrant colours, which become especially pronounced during breeding.
  • Temperament: These fish can be semi-aggressive, particularly when they are in breeding mode.
  • Diet: As omnivores, they enjoy a diet consisting of pellets, flake food, and live food.
  • Care Level: Kribensis requires intermediate care and attention.
  • Insight: To promote a sense of territory, provide caves or hiding spots in your tank, as Kribensis tend to establish and defend their areas.

Balancing Fish Temperament and Compatibility

Now that you’ve met some potential aquatic inhabitants for your 40-gallon tank, let’s dig into the art of balancing fish temperament and compatibility. This is where practical insights can make a big impact.

Compatibility Considerations

When selecting fish for your tank, it’s vital to consider compatibility, especially when dealing with semi-aggressive species. Spend some time researching and monitoring the behaviours of your chosen species cautiously. Mixing schooling fish with solitary or territorial ones can be challenging but rewarding.

The Advantage of Tank Size

One practical advantage of a 40-gallon tank is that it offers additional space to manage aggressive behaviours. Larger tanks allow for better parting of incompatible personalities, reducing stress on your aquatic inhabitants.

The Significance of Plants and Decor in a 40-Gallon Tank

Now, let’s turn our attention to the artistic side of fish-keeping: plants and decor. These elements play a pivotal role in creating a captivating underwater world within your 40-gallon tank.

Aesthetic Appeal

Plants breathe life into your aquarium, establishing a natural and visually-grabbing environment for your fish. Consider introducing species like Amazon Swords or Java Moss to transform your tank into a lush and vibrant underwater landscape.

Suggested Plants

Consider adding Amazon Swords for their lush and vibrant appearance. Java Moss is another excellent choice, providing a natural look while offering a safe retreat for your fish.

The Importance of Structures

Especially if you have cichlids in your tank, structures like caves and driftwood are essential. These features serve as territorial markers and hiding spots, reducing aggression and stress among your fish.

Upholding Water Quality in a 40-gallon Tank

When it comes to maintaining water quality in a 40-gallon tank, there are a few practical insights I’ve learned through my own experience as an aquarist. This tank size, while manageable, does present some unique challenges and advantages that need to be considered for the health and longevity of your aquatic environment. Here are some practical insights into this crucial aspect of fish-keeping:

Challenges to Address

  • Smaller water volumes in a 40-gallon tank mean that fluctuations in water parameters can happen more rapidly.
  • Filtration and circulation are essential to maintain water quality.
  • Regular water changes and testing are crucial to prevent the accumulation of harmful toxins.

Advantages to Leverage

  • Smaller tanks are more manageable and easier to monitor compared to larger setups.
  • You can select filters and other equipment tailored specifically for a 40-gallon tank, enhancing your control over water quality.

Regular water change :

First and foremost, regular water changes are non-negotiable. In my opinion, this is the cornerstone of good aquarium maintenance. I recommend performing water changes every one to two weeks, replacing about 20-25% of the tank’s volume. This helps remove accumulated waste and excess nutrients, and keeps your water chemistry stable. When conducting water changes, be sure to treat the new water with a dechlorinate to remove chlorine and chloramine, which can harm your fish. Also, try to match the temperature of the replacement water to that of the tank to avoid shocking your fish.

Water Testing:

 Testing your water parameters is another essential aspect of water quality maintenance. Regular testing allows you to detect issues before they become critical. If you notice any parameter creeping out of the optimal range, take corrective action promptly.

Suitable Filtration: 

Choosing the right filtration system for your 40-gallon tank is critical. Consider a canister filter or a high-quality hang-on-back (HOB) filter with a suitable flow rate for your tank size. These filters provide efficient mechanical and biological filtration, helping to remove debris and convert harmful substances.


Adding a sponge filter as a secondary filtration system can be beneficial. Sponge filters are gentle on your fish and provide additional biological filtration. Plus, they’re cost-effective and easy to maintain.

Lastly, consider the benefits of adding an airstone or air pump. These help oxygenate the water, especially important if you have a heavily stocked tank. Adequate oxygen levels promote fish health and reduce the risk of issues like low oxygen-related stress.


A 40-gallon tank offers the perfect blend of size and practicality for both novice and experienced aquarists. It’s a canvas for your creativity and a haven for your aquatic companions. As you embark on this journey, remember to consider adult fish size, compatibility, and the significance of plants and decor in your decision-making process. Upholding water quality should always be a top priority.

So, go ahead and immerse yourself in the enchanting world of fish-keeping with your 40-gallon tank. With the right knowledge and care, you’ll create a thriving aquatic ecosystem that brings joy and fascination into your life for years to come.