Aquarium water with a yellow tint can be symptomatic of a problem with the water conditions. In other cases, it can be a good thing. You have to figure out what's causing it to determine your course of action.
Certain types of driftwood and bogwood can discolor aquarium water. These woods contain tannins, which have a yellow to orange color that will vary based on tannin concentration. Tannins lower water's pH. For some fish, this mimics their natural environment, and some fish keepers are OK with the aesthetic of tea-colored water. However, some species of fish need higher pH to thrive. Boiling driftwood reduces the amount of tannins it will release.
If you've recently set up the tank or you've just added substrate, you likely clouded the water. Clouding, caused by tiny bits of dust floating in the water, may appear a variety of colors. Before adding gravel or sand to a tank, thoroughly wash it to reduce clouding. Do this by putting your substrate in a bowl or bucket, then adding water, stiring the sand or gravel and pouring off the water. Repeat until the new water comes off clear. If you've already added your substrate, the problem will eventually take care of itself and probably won't hurt anything in the meantime. Water changes and filtration will eventually take care of this problem.
Blooms of free-floating algae can also discolor water. Such water usually looks greenish but can have a yellowish tint. Additionally, bacterial blooms may also change the water color. This is most common in new tanks that are still cycling, or aquariums in which a filter has broken down. These blooms usually do not hurt the fish but can be a sign of other problems in the tank. If you address the root cause, the bloom will usually end.
Controlling Free-Floating Micro-Organisms
Bacterial and algal blooms are probably the most concerning cause of discolored water, since they are often the herald of unhealthy water chemistry. If you see the water become discolored, test for ammonia and nitrite and perform a water change. Limit the lighting period to 12 hours if you have plants, or completely shut down the aquarium lights for a few days if you have fish. This prevents photosynthetic algae from getting the light they need to grow. These blooms are typically short-lived if you take steps to address the underlying causes.
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