Starting your first freshwater aquarium is a rewarding endeavor. Here's a step-by-step guide to help you get started:

1. Research and Choose Fish: Begin by researching and selecting suitable fish species for beginners. Opt for hardy, low-maintenance fish like guppies, platies, tetras, or danios.
2. Select the Right Tank: Choose an appropriately sized aquarium, typically a 20 to 30-gallon tank for beginners. Ensure it's placed on a level, sturdy surface away from direct sunlight and temperature fluctuations.
3. Gather Equipment: Acquire the necessary equipment, including a filter, heater, lighting, substrate, thermometer, and water conditioner.
4. Aquascape and Decorate: Create an attractive environment with live or artificial plants, rocks, driftwood, and decorations. These elements provide shelter and mimic a natural habitat.
5. Set Up the Aquarium: Add rinsed substrate to the tank and fill it with dechlorinated water, following the water conditioner's instructions.
6. Cycle the Aquarium: Before introducing fish, cycle the tank to establish a stable nitrogen cycle, a process that typically takes a few weeks.
7. Choose and Introduce Fish: Once the tank is cycled, carefully select your first fish and introduce them to the tank. Acclimate them to the water temperature and chemistry.
8. Feeding and Maintenance: Develop a feeding routine based on your fish's dietary needs and avoid overfeeding. Regularly test water parameters, and maintain water quality through partial water changes and filter maintenance.
9. Observe and Adjust: Keep a close watch on your fish for signs of stress or illness. Make adjustments to the tank as needed to ensure their well-being.
10. Enjoy and Learn: Take the time to enjoy your aquarium and continue learning about your fish. Aquarium keeping is an ongoing educational experience, and you'll discover more as you go along.

By following these steps, you can establish a beautiful and thriving aquatic environment for your fish.

In a freshwater aquarium, several peaceful fish can coexist harmoniously. Some popular choices include:

1. Tetras (e.g., Neon, Cardinal, Ember)
2. Guppies
3. Platies
4. Mollies
5. Corydoras Catfish
6. Rasboras (e.g., Harlequin, Chili)
7. Bristlenose Plecos
8. Danios
9. Swordtails
10. Angelfish (in larger tanks)

Remember to consider tank size, water parameters, and individual fish behavior for a successful community aquarium. Maintaining a peaceful environment in a freshwater aquarium means avoiding species that exhibit aggressive or territorial behavior. This includes some cichlids, bettas, tiger barbs, red-tailed sharks, and larger species like Oscars, which can disrupt the harmony of the tank due to their aggressive tendencies.

However, if your goal is to establish a harmonious aggressive fish aquarium, follow these steps:

1. Select Compatible Species: Research and choose aggressive fish species known to coexist relatively peacefully. For instance, larger cichlids, certain puffer species, or aggressive catfish can be compatible. Ensure they have similar size requirements and water parameter preferences.
2. Tank Size and Layout: Larger tanks help distribute territories, reducing territorial disputes. Provide a well-structured environment with hiding spots like rocks, caves, PVC pipes, and driftwood, which can serve as shelters and boundaries. This minimizes direct confrontations.
3. Feeding Strategy: Overcrowding at feeding times often triggers aggression. To prevent this, distribute food evenly around the tank or use feeding rings to ensure each fish gets its share without competitive aggression.
4. Water Quality: Maintain optimal water conditions through regular changes and efficient filtration. Clean, healthy water reduces stress and mitigates aggressive behavior.
5. Continuous Monitoring: Observe your fish's behavior closely. If you notice increased aggression or signs of stress, consider adjusting the tank layout, adding more hiding spots, or removing the most aggressive individuals to restore peace.
6. Consider Individual Temperaments: Keep in mind that even within aggressive species, individual temperaments can vary. Some fish may be more or less aggressive than others. Be prepared to isolate or rehome overly aggressive fish if necessary.

In summary, creating a harmonious aggressive fish aquarium requires careful planning, ongoing observation, and the willingness to adapt the tank environment to maintain a peaceful and healthy atmosphere for your fish.

The number of fish you can keep in a singular aquarium depends on several factors to ensure the health and well-being of the fish and the overall balance of the aquarium ecosystem. Here are some key considerations:

1. Tank Size: The size of your aquarium is one of the most critical factors. Larger tanks provide more space for fish and offer better water quality stability. As a general guideline, you should allocate at least one gallon of water per inch of fish. However, this is a rough estimate, and specific requirements can vary depending on the fish species.
2. Fish Species: Different fish species have different space and territorial requirements. Some are schooling fish and need to be kept in groups, while others are territorial and may not tolerate other fish in their space. Research the specific needs of the species you plan to keep.
3. Fish Size: Consider the adult size of your fish. Some species may start small but grow considerably larger as they mature. Plan for their eventual size to avoid overcrowding.
4. Behavior: Fish behavior plays a crucial role. Peaceful fish are less likely to engage in territorial disputes, while more aggressive or territorial species may require extra space and hiding spots to reduce stress.
5. Filtration and Water Parameters: The filtration capacity and water quality in your aquarium also affect stocking levels. Adequate filtration is necessary to handle the waste produced by the fish. Water quality parameters, such as pH, temperature, and ammonia levels, must be suitable for the fish species you keep.
6. Maintenance: Regular maintenance, including water changes and tank cleaning, is essential to keep the aquarium environment stable and healthy for the fish. Overstocked tanks require more frequent maintenance.
7. Compatibility: Ensure the fish you choose are compatible in terms of behavior, water parameters, and diet. Some fish may not coexist well with others, leading to conflicts and stress.
8. Aquascape and Hiding Spots: Creating a well-structured aquascape with hiding spots, plants, and decorations can reduce stress and aggression among fish. It also helps divide territories and provides shelter.
9. Experience and Observation: Your experience as an aquarist matters. Novices should start with fewer fish and gain experience before considering a higher stocking level. Regular observation of fish behavior helps you identify and address any issues promptly.

In conclusion, there isn't a one-size-fits-all answer to how many fish you can keep in an aquarium. It depends on various factors, and careful planning, research, and monitoring are essential for a successful and healthy aquarium. Always prioritize the well-being of your fish over maximizing the number of fish in your tank.