Haven't Had The Heater On For A Week... Should I Be Worried!?

The chances are your heater wouldn't have been on during the last week even if it had been plugged in as the water would have been warmer that the level you have it set to.
I wouldn't worry this time, the weather will have kept the water warm enough. Just remember in winter  I've forgotten to plug my heater back in during winter and the temp has dropped to about 20 deg without harm. The fish did get a bit sluggish but they were OK once I turned the heater back on.

My heaters haven't been plugged in for over 2 months!

The water cant be cooler than the ambient temperature - Ive had fans on both tanks for about 3 weeks trying to stop the temperature rising too much   

Hmm I've always been a bit sceptical about my thermometer since it's a stick on one, maybe I should get a proper internal one and make sure my tank isn't too hot =/ All the fish etc seem happy enough and it still 'feels' about the right temperature it's always been. I was concerned my small tank would be the first to be too hot, but it seems to have been OK so far....

Quote from: SteveS on July 23, 2013, 12:00:37 AM

As water heats up considerably more slowly than air, the water most definitely can be cooler than the ambient temperature.

Yes initially the water would take a while to heat up to room temperature - obviously the larger the tank, the longer that will take - but the heatwave has been going on for a few weeks now, so I dont see how the water could cool down lower than the ambient temperature without intervention.

That's because you are thinking of a simple closed system, this is far from the case.  Some points to note:

• What you call "Ambient Temperature" is not constant.  Your room will be cooler during the night-time than during the day.  At night your tank will cool down. Probably!
• As water evaporates from your tank, the temperature falls. The process of changing from liquid state to gaseous state drains thermal energy from the water. The higher the rate of evaporation, the larger the cooling effect. Your fan cools things down using this effect.
• It takes much more energy to heat water than air

Interesting...... both my tanks have been 27c give or take 1/2 degree - and thats with windows open and fans on the tanks.

Whilst your tank has been reasonably constant at 27^oC, your room, "Ambient Temperature", has been, and probably still is, considerably higher.  This confirms that your tank temperature is lower than ambient!

During the day we have all windows and front door wide open, with 3 fans on full pelt to help circulate the cooler air...

This is your first mistake. Opening the doors and windows brings hotter air from outside into your room. Look at the advice that is being published to help the elderly and vulnerable cope with the heatwave. "Keep doors, windows and curtains closed". This applies especially to the new "super-insulated" homes. Don't take my word for it, google "coping with heatwave". You are just pulling hot air in from outside and heating up your rooms! Open your windows at night, when it is cooler outside, whether you believe it so or not!

They are just household fans that I have faced on the end glass where the filter intake pipe is. It helps, a TINY bit, to keep that end of the tank a little cooler and thus the filter churns the slightly cooler water round the whole tank.

This is your second mistake; A fan, household or industrial does not cool the air. In fact it will actually make the air hotter due... well never mind what it's due to, just take my word for it. All it does is blow it from one place to another. So you are blowing air onto your tank where it cools down by transferring heat energy to your glass; you then replace this cooler air with a fresh supply of hot air!

To use a fan (Yes a household fan) to cool your tank you need a different approach. Direct the fan across the surface of the water. This replaces the moist cool air above the surface with warmer drier air and hence encourages evaporation. Just like blowing on hot soup in a spoon; You don't blow on the bowl of the spoon, you blow on the surface of the soup! To see the size of this effect, dip two fingers in the warm water of your tank and place your hand in front of your (household) fan. You should, very quickly, notice a sizeable drop in temperature!

I think there's been enough energy the last few weeks to heat an indoor swimming pool

If the pool is deeper than 36" or so, it will never heat without artificial assistance.

though my initial statement was that I fail to see how it can be so, without intervention.

No, your initial statement was

The water cant be cooler than the ambient temperature - Ive had fans on both tanks for about 3 weeks trying to stop the temperature rising too much 

If you stop and reason, the statement gives rise to a logical contradiction.  If the water can never be cooler than the air temperature.  Any excess heat in the air is supposedly transferred to the water. Where else can it go? It follows, logically, that the air can never be cooler than the water temperature.

They are both the same temperature? Always? Really?

OK - stop it right now, kids. Play nicely, or I'll come round and knock your heads together!!

Well, everybody – this is a time I cannot resist stepping in – sorry!
First of all – BOTH Chuclett and Steve are ABSOLUTE correct! In their own approach to the topic.
Jesnon and Colin – this is NO harm in an argument! Jesnon, you should not feel guilty. Colin – being nice all the time can (in theory) bring more bad than good (example – if your close friend is suffering from a, say, alcohol addiction – is it better to be “nice” and pretend you don’t notice or is it better for your friend’s sake to insist they seek help?)
Now to the “core” – as far as I understand it – feel free to dismiss what I say altogether or argue it!
Let’s take the basic parameters: a house in the warm climate (it is a bungalow, is not Chuclett? – this is important for my deliberations...). So, the temperature at day time is, say, 27 degrees C and it is, say, 24 degrees C at night. The house is “modern insulated” so it does absorb whatever heat is available and does not part with this heat easily or quickly... This sort of weather stays  put for some weeks... The house also has: an oven and a cooker (constantly used to make meals), a TV (also used), electric powered fans (also used in an attempt to keep cool), a fridge and a freezer (both in constant use), potential use of an iron, a hair dryer, constant use of lighting when it is dark and plus the fish tanks – with their lighting, filtration and (now not that much used if at all) heaters... Oh, yes, another thing – the people living in the said bungalow are reluctant to leave the windows open at night because of safety reasons.
So, what we have? A house which does not have an “upwards” flow of air inside ( a bungalow in comparison to a two storey house where a natural flow of a hotter air upwards and out can be achieved), a few weeks of very warm weather and a restriction on taking the cooler air in at night?
I think the answer is obvious! Although all the theoretical notions of Steve are ABSOLUTELY correct, there is a reality which steps in. Water, indeed, heats up much slower than air – equally it cools down much slower than air as well. The house is exposed to all those appliances kicking off heat and does not have a chance to cool down at night (windows closed and it is a modern insulated one as well). The aquariums within such a house will take their time to reach the “ambient” temperature but being filled with water they will be also slower to cool down... I can absolutely believe that bearing everything above in mind, the water temperature in the said tanks has reached (and probably exceeded) the ambient temperature outside at a day time! As they will be slower to cool anyway (water) evening use of lighting, cookers, etc. inside the house and the fact that the windows are all shut, will not allow the water in the fish tanks to cool during the night...
And also, by the way – a pool of 36’’ deep will NEVER heat up to the ambient temperature?! It depends... If the pool is located in, say the Azores, where the air temperature is a constant +25 all year round year after year – I am sure that a pool of ANY depth there will be about a degree or so cooler than the ambient temperature  - simply because of higher density of the water. However, there will be times when this pool will be hotter than the air temperature...
Anyway, “science” aside – again: BOTH Steve and Chuclett are right. I understand that Chucklett is just trying a practical side of this trying to keep cooler and keep her tanks from overheating whilst Steve – in his usual manner – is going into a theoretical part of the question.
I don’t think anybody is upset or holding anything against each other – am I right???

As long as you guys are just having an interesting debate I'm happy!

Had to google "Strawman argument"  Never heard that term before!

As Natalia only passes through from time-to-time, Im guessing that she read through the whole thread in one go and thus read your post out of context. Afterall, Jesnon got the impression that a heated argument was brewing (maybe it was from SteveS point of view?  ). It has been mentioned before how easy it can be to misconstrew the manner in which something is said in cyberworld, albeit you added a cheesey grin or two in order to try to show you weren't being vindictive.

But hey, nobodys been hurt.... I dont think