If you've ever inhaled a lungful of pollutants while driving behind a large truck or bus, you can imagine life in an unhealthy aquarium. Water quality problems, such as ammonia 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.

You may think your fish enjoy the company, but an overcrowded aquarium can lead to massive pollution. Fish feces, decaying plants and algae, and uneaten food particles release ammonia as they break down. New aquariums often experience high ammonia levels because they lack the good nitrifying bacteria. It can take up to six weeks for a new aquarium to reach acceptable levels of ammonia and nitrifying bacteria.

Some symptoms of ammonia toxicity are: fish gasping for breath at the water surface, purple or red gills, lethargy and loss of appetite. Ammonia toxicity can happen suddenly or over several days. As soon as you notice a change in your fish, test the ammonia level as a first step toward diagnosing the problem.

Maintaining healthy levels of nitrifying bacteria will keep ammonia under control, but it takes time for the bacteria population to grow. If you start with too many fish in the tank, the naturally occurring bacteria won't be able to keep up with waste production. It's best to introduce only one or two hardy fish during the first month of the aquarium's operation.

Older aquariums typically contain enough nitrifying bacteria to handle ammonia output. Be sure to maintain a regular water and filter changing schedule, and test the water weekly.

Buy a water testing kit for weekly monitoring. Any ammonia value exceeding 0.06 ppm (parts per million) can be toxic. Your local PETCO or fish veterinarian also can test your water and offer advice on keeping ammonia under control.

If your tank's ammonia level is too high, change 25-50% of the water immediately to dilute the ammonia. You may need to change the water several times within a short period of time to reduce the ammonia level to less than 0.06 ppm. Continue monitoring the pH and water temperature in order to keep them as stable as possible; otherwise, the problem could be compounded.

You can also purchase a chemical to neutralize the ammonia. Consult with an aquarium specialist for guidance.

After you reduce the ammonia to an acceptable level, determine what may have caused the problem. Overcrowding and overfeeding are common culprits. Overgrown plant and algae populations also can boost ammonia, because new greenery will eventually decay and pollute your tank.

Remember: Moderation is the key to establishing a healthy aquarium ecosystem. Start with just a few fish and keep the population low even after the levels are stabilized; otherwise your quiet companions may quickly overwhelm your system.

Too many tankmates, overfeeding, poor filtration and an unclean tank can also 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. Some symptoms of nitrite poisoning are fish gasping at the surface of the water for air, listlessness, and tan or brown gills. Test your tank water to detect nitrite toxicity, or enlist a qualified veterinarian's help. Blood samples from nitrite-poisoned fish are typically brown.

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. Perform an immediate 25 - 50% water change and increase aeration. Reduce feeding and do not add new fish. If you have fish that can tolerate low salinity levels, consult with an aquarium specialist on the appropriate amounts of aquariums salts to use as an additional form of treatment. 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.

Your tap water contains chlorine or chloramines to kill such bacterial contaminants as E. coli. But these chemicals are deadly to fish, and you must remove them before turning your friends loose in their tank.

Chlorine will evaporate from standing water in two to three days; unfortunately, this is not true for chloramines. A chemical treatment must be used to remove chloramines from the water A test kit will tell you when the chemicals are gone. You can remove both chlorine and chloramines immediately with a commercial dechlorinating product. However, dechlorinating products will remove the chlorine portion of chloramines resulting in the release of ammonia so make sure the product you are using also removes ammonia. Use extra caution when setting up a new tank, because inactivated chloramines (though not chlorine) can release ammonia. Tanks with established filtration systems can quickly convert ammonia to a harmless nitrate molecule, but new tanks can't handle the workload.

Even the limited chlorine exposure from adding small amounts of tap water to an established aquarium on a long-term basis can damage gill tissue and lead to death.

Any detectable level of chlorine is too much. If you detect chlorine in your tank water, immediately treat your water as appropriate and reevaluate your aquarium maintenance routine.

The pH of a substance relates to the amount of hydrogen ion present. The pH scale measures pH from 1 to 14. A pH of 7 is neutral. Values of less than 7 are acidic and those greater than 7 are alkaline.

You don't need to understand the chemistry involved, but you should know the parameters affecting your finned friends. Most fish can adapt to life within a specific pH range. They'll tolerate gradual changes, but pushing the limits can cause illness.

When the pH drops, fish may display abnormal swim patterns and behavior. A change in pH greater than +/- 0.3 in a 24-hour period can stress fish and make them more susceptible diseases; therefore, any change in appearance may be due to something other than the pH level. Weekly water changes can help restore low pH balances to acceptable levels for your fish. Mineral leaching from limestone rocks or coral may cause alkaline increases. Adjust your tap water's pH according to your pets' preferred range. Commercial pH buffers can help. Then monitor the pH weekly. If it fluctuates, a simple water change may get it back into balance. However, if your fish do get sick, they may need other treatments.