Water Changes (Cont.)
When conducting a water change on an established aquarium, you should try to match the TDS and pH of the water you are replacing. Matching the TDS is more critical than pH, but ignoring pH can also lead to fish mortality.
When changing the water of the aquarium you should be within 100 ppm TDS after completing the water change. TDS can be adjusted after a water change if necessary. If a radical change in TDS is allowed after a water change, osmotic shock can potentially occur in fish, which can lead to fish mortality if not corrected.
In an established aquarium, pH will drop as nitrate increases. Adding sodium bicarbonate (baking soda) over the course of a week to bring the pH up gradually will help fish make the transition to a higher pH that will typically occur after a water change. Conducting small (25%) partial water changes daily over a week can also help fish adjust to the higher pH slowly. Fish mortality is common when aquarist do complete or near complete water change on an aquarium that has not had its water changed in several months, and the pH has been allowed to get very acidic (due to extremely high nitrate level) for an extended period.
The consistency of water temperature is often given over stated importance when doing a water change. In natural waters, 5 to 10°F (2 to 5°C) difference in temperature is common in different areas of rivers, streams, creeks, ponds and lakes that fish traverse daily. Fish are much more tolerant of temperature change than is often stated in aquarium publications. When a water change is made the temperature of new water should be within 5°F (2°C) of water in the aquarium.
If you have a balanced aquarium (nitrate does not accumulate over time), which is very rare, you cannot discount making water changes. Adding top off water over time can raise the TDS. Fish food also introduces additional TDS. For soft water aquarium, regular scheduled water changes should be done to keep the TDS low. An aquarium with a lot of live plants may require additional elements added on a schedule basis as plants will take up minerals as they grow.