🐟 How Fish Breathe: Vet Science Explained for 2025 🐠💨

By Dr Duncan Houston BVSc – Dive into the vet-backed science of how fish breathe, and learn what every aquarist needs to know in 2025!

🌊 The Basics: How Do Fish Breathe Underwater?

Fish breathe using specialized organs called gills located on either side of their head, under a protective flap called the operculum. These gills allow fish to extract oxygen from water and expel carbon dioxide—just like our lungs, but adapted to a much denser medium. 🫁💧

Water contains far less oxygen than air (~8–10 mL/L compared to 210 mL/L in air), so fish must use a highly efficient system to survive. Their gills, especially the gill filaments and lamellae, are built for maximum surface area and exposure to oxygen-rich water. 🌐🐠

🔄 The Breathing Process Step-by-Step

1. Fish open their mouths to allow water to flow in.
2. The water passes over the gill arches, lined with filaments and tiny lamellae.
3. Blood inside the lamellae flows in the opposite direction of the water—a mechanism called counter-current exchange.
4. Oxygen diffuses into the bloodstream, and carbon dioxide exits into the water.
5. The water then exits under the operculum. 💨🐟

💡 What is Counter-Current Exchange?

This is the secret sauce of fish breathing! By flowing in opposite directions, the water and blood create a gradient that maximizes oxygen uptake. Instead of oxygen only diffusing early on, it diffuses along the entire length of the gill filament, increasing efficiency up to 90%! 🧠🔬

🧬 Anatomy of Fish Gills

• Gill Arches: Bony structures that support gill tissues.
• Gill Rakers: Filter debris and particles out of incoming water.
• Gill Filaments: Long, thin structures where most gas exchange occurs.
• Lamellae: Microscopic folds loaded with capillaries for oxygen transfer. 🩸🔬

🌿 More Than Breathing: Other Functions of Gills

• Osmoregulation: Regulating salt and water balance in freshwater or marine environments.
• Ammonia excretion: Gills help remove nitrogen waste, working alongside the kidneys.
• pH regulation: Helping maintain internal acid-base balance. 🧪🌡️

🐠 Special Breathing Adaptations

Not all fish breathe the same way. Some species have evolved special techniques or structures to help them survive in tough conditions:

• Labyrinth organs in bettas and gouramis let them breathe air directly.
• Lungfish have primitive lungs and can breathe air during droughts.
• Sharks and rays often use multiple gill slits and may rely on ram ventilation (swimming to push water through). 🦈🌬️

🚨 Signs of Breathing Trouble in Aquarium Fish

Fish showing breathing distress may exhibit:

• Gasping at the surface
• Rapid gill movement
• Lethargy or sinking behavior
• Staying near water outlets or bubblers
• Loss of appetite

These signs indicate low oxygen levels or gill dysfunction, often caused by poor water quality, temperature spikes, or disease. ⚠️💧

🧪 Water Quality and Breathing

Keeping your water clean and oxygenated is critical:

• Ideal dissolved oxygen (DO) levels: 7–9 ppm
• Keep ammonia & nitrites at 0 ppm
• Monitor pH: abrupt shifts can harm gills
• Ensure proper water flow and surface agitation

Use test kits weekly to avoid hidden problems. 🌡️🔍

🩺 Veterinary Care Tips for Breathing Health

As a vet, I’ve seen too many preventable fish deaths due to hypoxia or gill infections. Here are my top 2025 care tips:

• Upgrade aeration: Air stones, sponge filters, and waterfall features enhance oxygenation.
• Avoid overstocking: More fish = faster oxygen depletion.
• Quarantine new fish: Prevents spread of parasites that can damage gills.
• Medicate wisely: Gill flukes and bacterial gill disease require vet-approved treatments like Praziquantel or Kanamycin. 💊🔬
• Watch temperature: Warmer water holds less oxygen. Keep species-appropriate temps.

🌟 Species Insights: Who Needs the Most Air?

• Goldfish: Produce lots of waste—need high oxygen & filtration.
• Discus & tetras: Thrive in well-aerated, soft water tanks.
• Bettas: Use labyrinth organ but still need clean water!
• Catfish & loaches: Bottom dwellers need flow to prevent low-oxygen zones. 🐡🐠

✅ Vet-Approved Aquarium Tips for 2025

• Install an air pump: Especially for tanks over 10 gallons or in hot climates.
• Check your filter: Ensure it’s circulating water properly and not clogged.
• Use plants: Live aquatic plants help oxygenate—especially during daylight hours.
• Try Ask A Vet’s AquaCare line: Natural water enhancers that support gill health and oxygen balance.
• Download the Ask A Vet app: Chat with a fish-savvy vet instantly for diagnostics and treatment help. 📲🐟

🔁 Can Fish Drown?

Yes—if water is too low in oxygen, or if their gills are damaged, fish can “suffocate” or “drown.” Common causes include:

• Power outages (loss of filter/aeration)
• Overheating (warm water = less oxygen)
• Diseases like gill flukes or columnaris
• Algae die-offs using oxygen at night 🌑🧫

🎯 Summary: What Every Aquarist Should Know

Fish breathe by passing water over their gills, using an advanced counter-current system to absorb oxygen. Healthy gills = happy fish! By maintaining excellent water quality, proper oxygenation, and observing your fish daily, you can prevent breathing issues and ensure your aquatic pets thrive. 🐠❤️

🔗 Ask A Vet for Help

Concerned about your fish's breathing? The Ask A Vet app offers instant access to aquatic veterinarians for help with gill issues, medications, and tank troubleshooting. With tailored plans, diagnostics, and our exclusive AquaCare support line, your fish are always in good hands. Download now for peace of mind and healthier fins! 🐟📲💙