Imagine this, you fetch a glass of filtered water ready to quench your thirst but alas! You are met by a sickening fishy smell or taste. You decide to check out other faucets, and to your utter amazement, the water is odorless and tasteless. Now you are left scratching your head, wondering why your filtered water has a distinct fish taste while the rest of the unfiltered water is just OK.

If your filtered water tastes like fish, there is a good chance you are using an ion-exchange filter. Your filter might be using a chemical process called ion exchange. The ions usually produce a by-product called trimethylamine, an organic compound that smells like rotten fish. Naturally occurring organic materials can also produce this flavor in water.

Keep reading to find out more on this problem and the possible solutions and alternatives.

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Where does the fishy smell come from?

Some water filters use activated carbon to get rid of contaminants from water. Carbon’s porous qualities make it a good choice for water filtration because it easily absorbs contaminants from the water.

However, the carbon filters cannot absorb 100% of the contaminants from the water, which is why ion exchange is sometimes used. Through ion exchange, dissolved ions and metals are removed from the water and replaced with ions of similar electrical charge.

Ion exchange filters employ a resin that has a network of polymers with various exchange sites and each site either has cations or anions.

Here is how it works:

Cations have a positive charge, while anions have a negative charge.
The cation is usually hydrogen (H⁺) while the anion is hydroxide (OH^–)
At the resin the ions removes impurities’ ions and exchanges it for H⁺or OH^–

When the two combine they form water.

H⁺ + OH^-= H₂O

To put it plainly, the resin removes contaminants and replaces them with water molecules. But it is not that simple. See, after a while, the ions used grow old and decompose. When cations decompose, they do not cause any harm. However, it’s a different story for anions.

As a result of decomposition, the anions produce trimethylamine as a by-product. This is the same product produced by decomposing fish flesh. So the next time you encounter filtered water that smells like fish, don’t think you have a dead fish in your plumbing –you just have an aged ion exchange filter.

So what can you do about it? Keep reading to find out.

How to avoid the fishy smell and taste in filtered water?

You should regularly replace your filter cartridges. Check the usage instructions to know after how long you should replace your filter. If not sure, just replace it after every 3-6 months, depending on how much water you filter.
Trimethylamine is strenuous to clean. If you replace your filter late, your solution is to disassemble your filter system and clean it with soap. Do not use bleach. Use paper towels to dry it, repeat several times, and reassemble with the new filter.
Avoid using hot water with an ion exchange filter since they degrade faster when exposed to heat.

Why does my tap water smell like fish?

Apart from a faulty ion exchange filter, there are several other factors that might contribute to your water tasting like fish.

The main cause of a fishy smell in your tap water is natural organic materials found in your water source. Metal elements like Barium which exists as ores can get into your water, causing a foul smell. The EPA has set standards to regulate the levels of Barium in public water. Cadmium comes from industrial waste or fertilizers.

The fishy smell could also be from a combination of chlorine and ammonia. The two are used as disinfectants in water treatment. Chloramine can sometimes make your water smell fishy.

Lastly, if you find yourself in the middle of summer with smelly tap water, you have fallen victim to algae bloom. Algae bloom in summer due to ample sunlight and warm water. People sensitive to musty or mildew smells are likely to notice. Not to worry, the smell does not indicate any health hazards. A good idea is to let your water supplier know about it and ask for more details.

Filters that use ion-exchange produce purified water. However, the anions present in the resin decomposes over time giving out trimethylamine which is the same compound produced by decomposing fish. This is the reason why your filtered water smells and tasted like fish. You can remedy this by changing the filters often.

But you may want to check to ensure the problem is actually caused by a faulty water filter. For instance, if you do not use an ion exchange filter, there is a good chance the fishy taste is coming from some other source.