Q: The water in my goldfish pond has turned a very dark brown color. What can I do to filter it out?
A: Is the water tea-colored but clear, or is it cloudy and brown? The coloring comes from tannins. The breakdown of leaves in water colors the water brown, like tea leaves in a tea bag. There are tannins in tree leaves, acorns, and branches, especially oak and hickory leaves.
We have three goals here. We need to make the water clear; we need to remove the source of the brown water; we need to prevent it from happening again. We can do the first two at the same time.
First, drain as much of the water out of the pond as you can. You can drain it halfway and leave the fish in, or you can place the fish in a tub and drain out most of the water. Filling the pond with clear water will dilute the problem of brown water away.
While the pond is empty or mostly empty, vacuum up the leaves and sludge off the bottom of the pond and any shelves. That gets rid of the current source of decaying organic matter that is causing the brown water.
If there are a lot more tree leaves that can fall or blow into the pond, cover the pond with a net. If there are only large leaves, a cheap plastic net with large holes can be a quick solution. These nets are often considered to be disposable because the leaves are hard to get out of the net and the nets tear easily.
I use a Nycon Big Top pond cover. Mine is 20 years old. It is a cloth net that has small enough holes that most honeylocust leaflets don’t make it into the pond. I can use a leaf blower to blow the leaves off the net if too many fall at once. It comes with poles, heavy-duty grommets, and stakes. It is easy to set up and take down.
If your pond has a skimmer to remove floating surface debris, don’t let leaves fill the filter net and sit for days at a time. The net will act as a tea bag, and the leaves will turn the water brown. Empty the skimmer filter net daily.
If the pond is small, you may try adding a bag of activated charcoal to the filter system. The charcoal can remove the tannins, but it takes a lot, and it has to be replenished regularly. In large ponds, it is easier to keep the leaves out of the pond in the first place by using a net to cover the pond.
The color itself will not harm the fish, but the tannins can turn the water acidic, and that can be a problem. Also, the bacteria that break down the organic matter to release the tannins consume oxygen in the water. A lot of bacteria consuming a lot of decaying organic matter can remove enough oxygen from the water to kill the fish. When the oxygen level in the water is low, other bacteria that decay organic matter in these conditions release sulfur compounds that can also kill the fish.
Tannins in a pond are not good, but tannins have beneficial uses. They are used to tan skins into leather. That is why it is called tanning. Tannins are part of the chemicals that color coffee, tea, and wine.
Pond filters will not take out the coloring from tannins, but if the pond is cloudy and brown, filters are useful. Are the particles from soil that is eroding into the water or from brown algae or from some other source? If the particles are too small for the filter material, can you use better filters? Filter material that can remove fine particles may clog quickly and need to be cleaned regularly. Adding fish-friendly flocculants that will clump the small particles together into larger particles can make normal filter material work better.
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