If you've ever inhaled a lungful of pollutants while driving behind a large truck or bus, then you can imagine life in an unhealthy aquarium. Water quality problems, such as nitrite toxicity, can cause disease and even death, so monitoring your tank's water is the most important step you can take to keep your fish happy and healthy.

Too many tankmates, overfeeding, inadequate biological filtration or a drastic change in water can lead to nitrite toxicity. While nitrosomonas bacteria convert ammonia to nitrites, their nitrobacter cohorts must convert these nitrites into nitrates. Problems occur when they can't do this.

Nitrite toxicity is similar to carbon monoxide poisoning and just as deadly. Excess nitrites prevent red blood cells from transporting needed oxygen. Affected fish experience oxygen starvation, or hypoxia, and float near air bubbles or the water's surface, desperately gasping for air.

Test your tank water to detect nitrite toxicity. If nitrite toxicity is the culprit in a fish's death, take immediate action or other fish will die - and the big fish usually go first because they demand more oxygen. Consult a qualified veterinarian or aquatic specialist for guidance and treatment options.

Again, prevention is key. Don't fully stock your aquarium for at least 30 days after setup, and don't overcrowd or overfeed your fish. Do schedule weekly water changes to maintain filter efficiency.