This week I had a semi-idiot moment (but still a lesson learned in the process) when trying to reduce the current water temperature in my aquarium momentarily (per another experiment of mine). Basically I have been told that when doing a water change you should keep the temperature of the new water within 3-5F of the aquariums current water temperature (as to prevent things like 'Ich' from happening). However, if you have a 79F tank, change 25% of the water (in say a 20 gallon aquarium), and add in 74F (which is 5F lower than the current water temp) water to replace that 25%... guess what, the aquarium water only dropped 1.25F degree in combination with the current tank water temperature (which just wasn't going to do because I wanted the overall water to become 74F momentarily and not 77.75F).
As a side note the upside to adding in cooler water than the tanks current temperature is an increase in the waters oxygen level (which is why I feel when you do a water change to always add in cooler water than the tanks current temp). Did you know water at 77F/25C has an oxygen solubility level of 8.6mg/L? Which means that the average oxygen level in a typical indoor freshwater aquarium is probably going to be around 8-9mg/L. Water at 32F/0C which is 'freezing point' has an O2 level of 14.6mg/L, while water at 212F/100C which is 'boiling point' has an O2 level of 0.
To break it down if I did a 50% (1/2 the tanks water) water change I would need to add in 69F water to my 79F for it to average 74F, which is a 10F temperature difference from the tanks current temp. So if I am only changing 25% (1/4 the tanks water) of the water I would need the new water to be 15F below 79F which would be 64F.
Now you may be saying that colder water is denser than warmer water (thus meaning the cold water would sink to the bottom of the tank) and that 'would' be problematic for aquarium inhabitants. However, when you add cool water to warm water the two temperatures combine together (because heat rises and cold sinks), but if you added hot water to cool water (thus reversing the order) then the two would take time to merge together (the later case could be problematic for aquarium inhabitants). Don't believe me? Than checkout this video on water density. So, if you happen to be using a tube to fill cooler water back into your aquarium you'll want this tube to be closer to the waters surface and not at the base of the tank in order to allow the two water temperatures to better merge together (unless of course you have other intentions for doing so).
As a side note the upside to adding in cooler water than the tanks current temperature is an increase in the waters oxygen level (which is why I feel when you do a water change to always add in cooler water than the tanks current temp). Did you know water at 77F/25C has an oxygen solubility level of 8.6mg/L? Which means that the average oxygen level in a typical indoor freshwater aquarium is probably going to be around 8-9mg/L. Water at 32F/0C which is 'freezing point' has an O2 level of 14.6mg/L, while water at 212F/100C which is 'boiling point' has an O2 level of 0.
To break it down if I did a 50% (1/2 the tanks water) water change I would need to add in 69F water to my 79F for it to average 74F, which is a 10F temperature difference from the tanks current temp. So if I am only changing 25% (1/4 the tanks water) of the water I would need the new water to be 15F below 79F which would be 64F.
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