5The fish tank in a public aquarium holds 420 liters of water. If 60 liters are removed each day for cleaning and maintenance, and 25 liters are refilled daily from a sustainable source, how many liters of water will remain after 8 days if no additional water is added after the initial filling? - Parker Core Knowledge

Understanding the Context

How Water Is Managed in a Public Aquarium: A Closer Look at Daily Changes

Maintaining a pristine and stable environment in a public aquarium requires careful water management. Take the example of a large public aquatic exhibit holding 420 liters of water. To ensure water quality remains optimal, aquarium staff remove 60 liters of water daily for deep cleaning, filtration maintenance, and water sampling. However, to sustain a sustainable model, they replenish 25 liters each day from a responsible water source.

Let’s analyze the water balance over an 8-day period to understand how much water remains in the tank under these conditions—assuming no extra water is added beyond the initial fill (420 liters) and the daily removal-and-refill cycle.

Key Insights

Daily Net Water Loss: What Happens Each Day?

Each day:

• Water removed: 60 liters
  
• Water refilled: 25 liters

Net daily change in water volume:
60 liters removed – 25 liters added = –35 liters per day

Over 8 days, total daily loss multiplies:
35 liters/day × 8 days = 280 liters lost

Final Thoughts

Total Water Remaining After 8 Days

Start with an initial tank volume of 420 liters.
Subtract the cumulative loss after 8 days:
420 liters – 280 liters = 140 liters remaining

What This Means for Aquarium Maintenance

While 140 liters may seem low, in reality, public aquariums use partial water changes combined with large tank sizes and efficient filtration to maintain stable, healthy environments. The removal of 60 liters daily is balanced by meticulous cleaning schedules and sustainable refills, minimizing stress on aquatic life.

This scenario illustrates that thoughtful water management ensures tanks stay functional and safe—even with significant daily water exchange. Regular monitoring, water quality testing, and sustainable sourcing remain key to preserving marine ecosystems in public displays.