Does Coffee Kill Fish? (Essential Reading)

My young niece put her mum’s iPhone in the bath and instant coffee granules in the goldfish bowl. While the former is harmful to her mom’s social life, is the latter harmful to the fish?

The caffeine in coffee reacts with the bacteria in your fish tank to change the ph and oxygen levels and increase ammonia. In small amounts, this is not harmful to fish, but large amounts can definitely kill them.

Let’s dive down (pun intended) into the topic of coffee killing fish in more detail, and discover just how harmful caffeine can be to our colorful and gilled friends…

Is Caffeine Poisonous to Fish?

It happens: your kids have decided to “feed the fish” or “top up the water” in the fish tank with your morning cup of coffee. Will the caffeine in the coffee poison the fish? The general consensus seems to be “No, it won’t kill your fish BUT”.

Let’s break down this answer. A relatively small amount of coffee tipped into a large-ish amount of water will be pretty diluted and shouldn’t harm your fish. However, coffee can alter the pH level of the water because it’s more acidic, especially if it contains milk. 

Fish breathe by taking in the free oxygen in the water and filtering it through their gills. Coffee doesn’t have the same level of oxygen as water, so the fish won’t be able to breathe as well.

So really, the effects will still be potentially damaging for the poor fish, even if the cup they get doused in contains decaf coffee. The liquid is still altering the pH and oxygen levels in the tank.

What can you do about this? Well, it’s all in the coffee: fish tank water ratio. If it’s a bit of coffee in a 75-gallon aquarium, the best option is to leave it, although maybe run the filter with some new charcoal.

If the ratio of coffee to water seems high (or the water looks darker), it’s time for a partial clean.

New to keeping fish at home? This helpful film talks about how to clean out your fish tank and change the water.

So, coffee can be harmful to fish in relatively large quantities, whether it’s caffeinated or not. However, does caffeine cause any specific problems?

There have been scientific studies carried out on the effect of caffeine pollution on the freshwater envionment, and we’ll take a closer look at those now.

What Does Caffeine Do to Fish?

Scientists have carried out research carried out into the possible effects of caffeine on freshwater life. In a 2009 study into the effects of caffeine on fish, caffeine was introduced into a tank and the water quality was monitored.

The scientists found an increased level of ammonia, which is not good news for fish.

It seems like caffeine can react with already-present bacteria to produce high levels of ammonia. This is toxic to fish in high quantities, and those fish taking part in the experiment were swiftly relocated when the high amount of ammonia was detected.

In conclusion: caffeine may not be toxic itself, but when it breaks down it can react with bacteria to produce lots of ammonia, which definitely is toxic. 

This is why we have to change our fish tank water periodically: if ammonia builds up, it creates toxic nitrates. Unlike our home aquariums, we can’t keep draining and refilling our rivers if the ammonia gets too high.

Are Brewed Coffee, Coffee Grinds & Instant Coffee All Harmful to Fish?

Any foreign item added to a fish tank has the potential to do harm to the residents.

While a small, very diluted quantity is probably OK (and please remember, we’re not veterinarians or marine biologists, so don’t take our word for it!), larger amounts could harm the delicate balance in the water.

Coffee alters the pH of the water and affects the oxygen levels. As we just learned, caffeine can also cause ammonia to be present in the water. So, given that, all three types of coffee can be harmful to fish.

We’ve heard of young kids deciding to feed the fish using instant coffee granules. Prevention is better than cure. Choose a fish tank with a lid, and make sure the aquarium feeding portal can’t be easily opened. Screw the coffee jar lids on super tightly.

You can also involve the kids in the fish feeding routine, then they’ll gain a better understanding of how to care (and how not to care) for their aquatic pets.

Is Caffeine Toxic to Other Aquatic Life?

There have been studies carried out recently into the impact of caffeine on aquatic life. The results aren’t encouraging, with caffeine emerging as a definite polluter of river and marine environments.

How has this happened? Well, not to put too fine a point on it, we consume and then pee out a considerable amount of caffeinated liquid.

In the US, 62% of us consume coffee every day. Most of us have three cups. That’s a lot of coffee. We drink our espressos and lattes, we use the bathroom, the caffeine passes from our system into the drainage system, and from there, some enters the waterways and seas.

But how does it bypass the sewage treatment works? Well, as we mentioned in another article, caffeine is one tough chemical, which hangs around long after the rest of the coffee has broken down. Not all sewage plants can deal with it.

Research into the toxic effect of caffeine on aquatic life concluded that caffeine has: “adverse impacts on aquatic organisms… inducing oxidative stress… neurotoxicity,  changing energy reserves and metabolic activity, affecting reproduction and development, and in some cases, causing mortality.”

More studies are being carried out to further discover the effects of caffeine on our environment, and what can be done to prevent it from entering our rivers and coastal waters.

Does the Smell of Coffee Attract Fish?

Weirdly yes, the smell of coffee attracts fish (although we can’t really imagine a remake of Jaws where they’re chucking coffee grounds over the side instead of chump).

Why is this? It’s thought that coffee stimulates the fish’s sense of smell, drawing them in. In fact, some anglers use coffee grounds as a lure when they’re fishing.

How do they do this? Well, the easiest way is to keep your fishing bait worms in a jar of used coffee grounds overnight, then the fish is attracted by the double-whammy of worm and coffee.

Apparently, most worms also like coffee (hence its popularity as an important compost pile addition).

Fish also love cheese, garlic, and alcohol (they have civilized tastes) but will swim away from bug spray, soap, and nicotine.