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I don't know where to find a chart but if there is one, it would only work for a tank of a particular volume since the critical factor is the proportion of water changed.

To drop the temperature by 10 degrees, you would need to do a water change of half the water with water that was 20 degrees lower, or a quarter of the water that was 40 degrees lower. You simply work it out by proportion, and the less water you change the greater the difference in temperature of the added water needs to be.

Do this fast and you're likely to induce temperature shock. Water is relatively stable in temperature, meaning it tends to change temperature slowly, and fish aren't well adapted to fast temperature swings. You're better off doing it in a couple of water changes rather than in one. There are a couple of other techniques for lowering the temperature a bit more slowly than a large water change. You can simply add ice blocks and allow them to melt slowly. Because there's a large temperature difference between the ice and the tank water, you need a much smaller volume of ice. You could also put the ice in a net bag of some kind and place it in the water flow from your filter. That will help distribute the cooled water around the tank, and the fact that the ice takes time to melt slows the speed of the temperature change which will help prevent the fish from being shocked. Another, similar, trick is to get empty large drink bottles, fill with water and freeze, then add them to the tank. When the ice in the bottle has melted, return the bottles to the freezer and replace with other frozen bottles. You can drop the last few degrees fast with a partial water change at lower temperature if you want a sudden snap to stimulate spawning.

I use the water bottle trick to help keep temperatures in my tank down in summer. My tank can get up around 32 degrees Celsius in summer, way over the 26 or 27 degrees Celsius I like to run it at. Adding frozen water bottles in mid to late morning as it starts to heat up helps to slow the temperature rise and keep the temp at a much more reasonable level, though I do allow it to climb a couple of degrees.

Also, if you have a heater in the tank, remember to lower its setting to the temperature you want to achieve, otherwise your heater is going to prevent you from ever getting there.

David Aiken

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