It is my first time to post a question here. Since it is rainy season in our country, the temperature in my fish tank is a bit lower than before. I want to increase the temperature in my fish tank but the problem is my fish tank do not have light apparatus and heating apparatus.

The only thing I can think of is to put a hot water in my fish tank with the fish in it.

2 opaline guoramis, 8 rosy barbs, 6 black tetras and 2 platys.

Is it safe or dangerous to my fishes?

Thanks a lot.


4 Answers 4


More dangerous than it is safe. Certainly very stressful.

Guoramis are quite difficult to keep and it is most likely that they'll suffer the most (you listed your fish in decreasing order of hardiness). If you put hot water into your tank, the temperature in certain areas will be extremely hot until the heat diffuses - basically, a pH spike, but with temperature. This is a water change that you'd be performing, which has its own connotations for chemicals and pH, but the temperature will only last for a little while.

I just don't see how it's worth all the stress to your fish. Aquarium heaters really aren't that expensive and are standard equipment, anyways.

If you really, really feel you need this, your safest bet might be to put a lamp near the tank, facing so that the light doesn't shine in the tank, but the heat from the lamp can be near the tank. Some bulbs are hotter than others and if you're going to do this, you need to keep a close eye on the temperature so you don't get into a situation where the water has heated to boiling temperature.


Well, yes, it is possible, but I wouldn't advice.

Boiling water changes water hardness.

Temporary hardness is mostly destroyed by mere boiling of water, when bicarbonates are decomposed, will produce insoluble carbonates or hydroxides, which are deposited as a crust at the bottom of vessel. source, related Wikipedia link

Boiling water can increase its pH:

boiling the water would also have removed any dissolved CO2. Dissolved CO2 will form a small amount of carbonic acid, which can significantly lower the pH of very soft water. The fresh water would then have a lower pH than boiled water. source

What about temperature?

If you pour hot water directly to the tank, you risk that a fish will swim just under the hot water. Maybe some dirt will raise up because of water moving the bottom, which will cause fish to think it is food or something else worth examining. Even if this water is getting cooler relatively fast, it is still hot and can harm the fish, if it will not manage to escape. Maybe consider inserting another tank (made of glass if you are able not to destroy this with temperature differences, or metal) in the aquarium, so there is no risk that any fish touches the hot water, but this take space of your tank.

You can calculate what amount of waters of what temperature you should mix to have the temperature of the sum, because it would be the weighted mean. So the final temperature T will be

T = (Tt * Vt + Th * Vh) / (Vt + Vh)

where Tt is the temperature in the tank of volume Vt , and Th is the temperature of hot water of volume Vh.

For example, you have a 100 l of water in your tank of 23 °C and you add 1 l of water of 98 °C this would lead to 101 l of ca. 23.7 °C. This would be true if the waters mix fully, however they will never do. The warmer part will get cooler because of air and glasses. In fact, you will never reach stable temperature of 23.7 °C in the whole aquarium.

I would agree with rib.usa that it is in fact not worth effort. The water will soon cool down because the air has lower temperature and you will need to repeat the process, probably just after an hour. But your tank does not have an infinite volume, so you need to take some water out. Every time you risk a stress for fish and you change water parameters, as shown above.

I would invest and buy a heater. They have thermostats and keeping right temperature is no effort from you. I think you can buy one for about 30-50 USD. If you are afraid of cost - I have 100 W heater which works about one third time a day, let's say 8 hours. This makes about 0.8 kWh a day, or 24 kWh a month. I don't know what is the price of energy in your country, in mine it is about 15-20 US cents. So the total monthly cost is about 5 USD. You can calculate yourself how much money would you spend to warm water by other means and how much time you are going to waste for preparing this water (time is money, too, remember).


A way to reduce the stress is to take a cup of the hot water, then several cups out of the tank itself, discarding one, and mix them completely before restoring it to the tank. You could also pull out water from the tank to heat, thus not having to worry about the water being 'new'.

If you either add enough tank water, or heat up the water less you can get the energy into the tank without having it shock the fish.

So for example, if you have 1 part water that is 50 degrees warmer, mixing it with 9 more parts from the tank will now just be 5 degrees warmer, and presumably not that risky for the fish. And you can just repeat as needed.


If you want to increase the water temperature, you can try filling a bottle with hot water and then gently immersing it into the aquarium.

You can hold the bottle and control the depth of immersion. This will cause the heat to gently spread through the aquarium. Keep monitoring the temperature of the aquarium with a thermometer.

Another alternative is to have an electric bulb (filament type) near the aquarium. You can control the temperature of the aquarium by adjusting between the electric bulb and the tank. Keep monitoring the temperature of the aquarium with a thermometer.

A better alternative is to purchase a heater which enables the maintenance of precise temperature with the help of a thermostat.


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