Aquarium FAQ: “Is Tap Water Okay For My Fish?”

Hey Everyone,

I’m happy to bring you this first installment of Aquarium Care FAQ – a new theSmileyFish series. I’ve been receiving a lot of emails from people who’ve asked for help with their new aquariums. So, I’ve decided to share both some of the most frequently asked aquarium care questions and the answers with the general public.

“Is tap water harmful to tropical fish?”

Yes; untreated tap water is harmful to all fish. Tap water contains chlorine, chloramines, phosphate, heavy metals, and other harmful impurities, all of which water utility companies add to drinking water to make it “safe” for human consumption.

While the human body can withstand these sanitizing poisons added to tap water, tropical fish cannot!

That’s partly because tropical fish have extremely thin skin which will burn when exposed to chlorine and chloramines.

This is why all tap water intended for aquarium use must be treated with something called water conditioner before being added to the aquarium. There are a variety of water conditioners available on the market and they will all effectively remove chlorine and chloramines.

Most water conditioners need a few minutes to take effect (some need more). When adding fresh water to an aquarium that’s already set up (into one that already has fish in it), I put the tap water into a clean bucket, then add the water conditioner, swirl the water around with my hand to ensure proper mixing, and then wait a few minutes before adding that water into the aquarium.

It’s important to note, however, that water conditioner will not remove phosphate from your tap water.

Phosphate is a compound that’s readily absorbed by problem algae. Phosphate, in effect, promotes problem algae blooms in the home aquarium.

So, if you want to go the extra mile by removing not only chlorine and chloramines, but also phosphate, you can opt to use RO (reverse osmosis) water for your aquarium.

RO water is pure H2O, so you don’t have to add water conditioner to RO water.

You can usually find Ro water at your neighborhood grocery store or even at your local aquarium shop …or you can make it yourself!

If you want to make your own RO water, you’ll need something called a RO unit – it’s basically a machine that uses a superfine semi-permeable membrane to physically separate “dirty” water from pure H2O.

I bought my RO unit for $240.00 (the picture above shows mine in its temporary bathroom setting). Yes; that was a bit of an investment, but the machine has paid for itself not only in financial terms, but also by cutting down potential maintenance caused by problem algae growth. In fact, when I first started using RO water, algae growth decreased by a whopping 90% and I haven’t seen any cyanobacteria (blue green algae) in ages! I’ve experienced no problems with the unit after years of operation.

Also, keep in mind that RO water is pure H2O. That means that it’s extremely soft and that it, therefore, cannot sustain high pH levels (for a detailed discussion of hardness/softness, buffering capacity, and pH see pages 30-32 of the Aquarium Care Book).

So, if you use RO water, and only RO water, you’ll have to add trace element solutions to compensate for the loss of beneficial compounds lost during the reverse osmosis process. Adding such solutions will also solve the potentially low pH problem.

Whether you use water conditioner or buy or make RO water, remember that the chlorine and chloramines in tap-water will quickly kill your tropical fish. So, tap-water always needs to be processed by one of the two ways outlined above.


One person commented on "Aquarium FAQ: “Is Tap Water Okay For My Fish?”"
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  • andrew says:

    I always put tap water into my tropical fish tank but I boil it in a huge pot and add tonic salts. and I’ve been doing this for 2 years now, and my fish seem happy swimming around except one died from popeye. but i think my fish are hardy because they are neon tetras and a black fish with a yellow tipped tail that eats algae and reproduces every day (lol). I also have 2 plecos, 2 snails, some danii’s and mixed breeds (lol). After getting some fish they all mated and had oddly mixed babies (lol).

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