Here are some common ways to determine if your aquarium filter media needs to be replaced:

1. Observe the duration of use: Each type of filter material has its own recommended service life. Usually, activated carbon lasts about 3-6 months, ceramic rings can last 6-12 months, and some high-quality filter materials may last longer.

2. Check water quality indicators: Regularly test your aquarium for levels of ammonia, nitrites, nitrates and other harmful substances. If these indicators are abnormally elevated, it may mean that the biological filtration ability of the filter media is decreasing and needs to be replaced.

3. Observe the condition of the filter media: Open the filter and examine the color, shape and texture of the filter media. If the filter media is darkened, changes shape or becomes loose in texture, it may be a sign of an aging microbial community or wear and tear on the physical filtration material, and replacement should be considered at this time.

4. Changes in bioburden: If there is an increase in the number of organisms in the aquarium, such as an increase in the number of fish or corals, then the burden on the filtration system will increase and the filter media may need to be changed more frequently to maintain water quality.

5. Water flow and clarity: If you notice that the water flow in your aquarium is decreasing or the water quality is becoming cloudy, it may be a sign that the filter media is clogged or the filtration efficiency is decreasing, and the filter media needs to be cleaned or replaced.

When replacing the filter media, it is recommended not to replace it all at once, but to replace it in batches so as not to disrupt the balance of the microbial community in the aquarium.