Before customizing an aquarium, you must first decide whether you want to keep saltwater or freshwater fish. These two types of fish are fundamentally different, offering distinct observational values. Below, I’ll outline their primary differences, starting with their essential distinction:

1. The most fundamental difference stems from their respective “liquid prisons.” Freshwater fish inhabit environments with salinity below 0.05%, while seawater averages a high salinity of 3.5%. This isn’t merely a matter of taste; it triggers a life-or-death battle for internal-external equilibrium—osmotic pressure regulation.

Freshwater fish have higher salt concentrations internally than their surroundings, causing water to constantly seep in. To counter this, they evolved powerful kidneys that maintain balance by excreting large volumes of dilute urine. Conversely, saltwater fish have lower salt concentrations internally than the ocean, leading to continuous water loss. They must actively drink water and expel excess salt through specialized “chlorine cells” in their gills. This intricate mechanism of intake and expulsion functions like a precision instrument designed by nature.

This fundamental difference shapes their physiological traits. Many marine fish require constant swimming to obtain oxygen, resulting in more developed muscles, while freshwater fish often have more agile bodies to adapt to varying water currents.

2. Due to environmental constraints, most freshwater fish exhibit limited color variation, predominantly in black, white, or red tones. From an aquarium perspective, this offers fewer decorative options. In contrast, the vast expanse of seawater has fostered greater evolutionary diversity among marine fish, resulting in more vibrant and varied colors. Combined with coral arrangements, they create truly stunning displays in aquariums.

3. From a breeding perspective, freshwater fish are simpler to maintain due to their water requirements. Saltwater fish, however, necessitate converting ordinary household water into seawater through artificial adjustment, with all water parameters needing to meet strict standards for successful breeding. This demands higher technical expertise, though saltwater fish also offer greater ornamental value.

Therefore, different fish species each have their merits. It ultimately depends on your personal preference and whether your skills can meet the higher demands of saltwater fishkeeping.