Everything looks clear and healthy from the outside, so you've relaxed your water-changing and water-testing schedules, trusting your filtration to maintain good water quality. What you can't see from the outside, however, is the slow and steady decline in water quality. You may be on your way to Old Tank Syndrome.
What is Old Tank Syndrome?
Though nitrate is not as toxic as ammonia or nitrite, high nitrate levels place chronic stress on fish. They become more susceptible to disease and demonstrate poor growth and color development. Extremely high nitrate levels threaten your entire aquarium system with a potential decline in pH as "old," nitrate-rich water loses its buffering capacity and becomes susceptible to the acidifying effects of decomposing organic waste materials. Eventually a pH crash may occur, which can be very damaging, if not lethal to fish.
How to prevent Old Tank Syndrome
If you think your freshwater aquarium demonstrates signs of Old Tank Syndrome, test your water right away. You can easily prevent nitrate buildup through regular water changes. If nitrate levels are high (above 60 ppm), perform a series of small water changes (no more than 25% per day) until it is lowered to acceptable levels (< 50 ppm). These water changes will also replenish lost carbonate ions (buffers) to help stabilize pH and create a healthy living environment. Be sure to use a bacterial additive, like Stress Zyme to maintain your population of beneficial bacteria.
Rather than combating the effects of Old Tank Syndrome, practice proper aquarium husbandry and perform routine maintenance. Insist upon regular aquarium maintenance, especially regular water changes and water testing.