Boiled water in the aquarium – is it worth using?

Over the years, many myths and misunderstandings have emerged regarding the use of boiled water in aquariums. Although fewer and fewer aquarists choose to use it in their home tanks, this topic continues to resurface in discussions on online forums. That’s likely because boiling water reduces its hardness. But does using boiled water in an aquarium actually make sense for this purpose? And more importantly is it economically viable? To answer these questions, I conducted a simple experiment. Join me in discovering whether boiled water can be an effective method of reducing aquarium water hardness.

How to reduce the hardness of aquarium water?

As I explained in the post Lime Deposits in the Aquarium and Biogenic Decalcification, there are two types of hardness in water: carbonate hardness and bicarbonate (temporary) hardness. Together, they make up the total hardness. In most Polish households, tap water is medium-hard to hard, while many fish species naturally inhabit soft waters. Therefore, we need to find a way to reduce the hardness of the water we add to our aquariums.

There are several ways to achieve this: using distilled water, demineralized water via ion exchange resins, water from a reverse osmosis (RO) filter, or boiled water also known as thermally softened water. At first glance, the latter method seems the simplest. But is boiled water truly a simple and economical way to soften aquarium water?

Thermal water softening – the experiment

For the tests, I used tap water from Gliwice in Upper Silesia undeniably among the hardest waters in Poland, mostly sourced from deep wells. The general hardness ranges between 21°n and 30°n. I measured the concentrations of magnesium and calcium our focus here, using the simplest method: titration analysis.

This is an accessible experiment for anyone with basic lab experience and minimal equipment. Although this method offers less precision than advanced analytical techniques, it’s sufficient for aquarium-related purposes. Titration works best with higher starting concentrations, so Gliwice tap water was ideal.

Initial Parameters of the Test Water

• GH: 24 °n
• KH: 13 °n
• Ca: 190 mg/l
• Mg: 36 mg/l

Required Equipment

• Two 1.5-liter pots with lids
• Two liters of tap water
• Electric stove
• Thermometer
• Five sample containers
• Electricity meter

Experiment description

One liter of 18°C water was poured into each of the two 1.5-liter pots. Both were covered with lids and brought to a boil. Once the temperature reached 100°C, it was maintained for 15 minutes. From one pot, five samples were collected at 1, 3, 5, 10, and 15 minutes after boiling started. These samples were cooled to room temperature.

Why use two pots? Because 50 ml of water was needed for each measurement of Mg²⁺ and Ca²⁺ ions, and each measurement was repeated three times, resulting in a loss of 150 ml from each pot. Losing 15% of the volume would significantly affect the results, as less water heats faster. That’s why I replenished the second pot after each sample.

As a control, I used RO/DI water produced in my shop. By definition, it’s completely free of hardness (dissolved salts), so all readings should be zero.

Results of the experiment

Sample	GH [°n]	KH [°n]	Ca [mg/l]	Mg [mg/l]
Tap water	24	13	190	36
Sample 1 (1 min*)	22	10	150	36
Sample 2 (3 min*)	20	8	145	35
Sample 3 (5 min*)	16	7	115	35
Sample 4 (10 min*)	14	6	100	33
Sample 5 (15 min*)	13	5	95	32
Water from the kettle	15	6	115	32
RO water	0	0	0	0

*Time measured from when the water reaches 100°C.

Boiling one pot of water used 0.37 kWh of electricity. The boiling process resulted in a 46% reduction in general hardness (GH), a 62% reduction in carbonate hardness (KH), a 50% reduction in calcium concentration, and an 11% reduction in magnesium concentration.

Conclusions from the experiment

As water boils, both general hardness (GH) and carbonate hardness (KH) decrease linearly. This happens due to the thermal decomposition of bicarbonates (KH components) into less soluble carbonates. These carbonates precipitate onto the surface of the pot, lowering calcium concentrations.

Magnesium carbonate, being more soluble, precipitates far less – only an 11% reduction. It likely adsorbs onto pre-deposited calcium carbonate, aiding its own precipitation.

Many factors influence the process of thermal softening. Crystallization (precipitation of CaCO₃ and MgCO₃) requires nucleation points such as scratches or imperfections in the pot’s surface. A newer pot may be less effective. Even a thin, invisible layer of calcium or magnesium salts already present in the pot can affect results. Variations in water temperature, experimental error, or miscalculations can also play a role.

In the table, you’ll see “water from the kettle”, another useful comparison. I made some tea during the experiment, which gave me the chance to take a bonus sample. The kettle hadn’t been descaled for 10 days, so a thick carbonate layer had formed. These act as nucleation points, accelerating further precipitation. Not surprisingly, the hardness dropped faster.

Let’s sum up: Does boiled water make sense in an aquarium?

What did we learn? To reduce the hardness of 1 liter of water by 50%, we consume 0.37 kWh of electricity. Multiply that by the cost of 1 kWh, and then by the weekly water change volume, 20, 50, or even 100 liters, and the numbers add up quickly. With large tanks, this matters… assuming you even have a pot big enough. Otherwise, it becomes time-consuming. Add to this the cost of water tests, since you never know the final parameters and must check them each time.

So, does boiled water in an aquarium make sense? Boiling removes bicarbonates, which are crucial pH buffers. When it comes to softening aquarium water, RO/DI water remains the best choice, it can be re-mineralized or mixed with tap water as needed.

Ultimately, the decision is up to the aquarist.

Michał Wyskiel