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When you do a water change how much temperature difference can there be between the tank and the water you put in?

i would advise trying to keep it as close as possible.

Is your tank planted?

IMO don't go too crazy trying to match temperatures. As was said, keep it as close as you realistically can - feel the tank water with your hand, then match it with the tap water. Being a few degrees off shouldn't hurt your fish.

Some fish even require a "blast" of cooler water to stimulate breeding. I've heard it suggested that adding water that's cooler than the tank water during water changes can lead to ich, but IMO and IME there's more to it than that - poor overall water quality and already stressed fish are more likely to blame than the addition of clean but cooler water.

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Back in the saddle!

I try to keep it within 1-2F but this can be difficult if you dont have a digital thermometer. I would suggest a digital thermometer like a wine thermometer as it serves duel purpases.

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I got a cheap thermometer that I keep in the water change bucket, I just try to get the temp as close as possible, not too cold or too warm, within 2 degrees or so. In a bigger tank it takes a really big difference to cause any problems but in smaller tanks it is much easier to affect the whole tank. Make sense? Using fingers isn't always reliable as they can be fooled.

"If you're afraid you'll make a mistake, you won't make anything."
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The hand method of testing water temp really isn't a good way to do it. Quite often the hand that's comparing temps is the one that's been in the tank to begin with, and if the room is below body temp your wet hand will cool off making the water in your water bucket feel warmer than it actually is :%)!.

Go with a thermometer, if you don't want to spend the money on two (or if you manage to melt yours no that never ever ever happened to me ) you can just transfer the one from your tank to the bucket and let it set in the bucket for 5 minutes or so to ajust.
1-2 degrees F is the most you should really go for, hardier fish will of course be more tolerant of changes. If you're noticing that the fish seem a bit stressed after a water change you might want to double check how different your temperatures are, otherwise dont stress on it too much.

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When I do a water change I heat the water up in a laundry trough. When the water actually goes into the tank I would say it could be well above the tank temp. The bigger the tank the less the problem. I would not add it to the tank if it was lower. When I check the thermometers I have noticed it might be about 1-2C up.

Before any one jumpes down on me I have been doing this for 35+ years and I think this would be well above many members here.

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It's better to try to keep the same temperature but 1 or 2 degreece wouldn't make much difference in big tanks.

If you have a small tank, try to keep the same temperature.


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Yup, my water changes go in at about 1-2 degrees f of variance. You can use a spare aquarium heater to match them up in the container before it goes in ,if youre worried.

Some people use drip systems to put water in so gradually it makes no difference. I suppose all it takes is a shelf, a bucket , some tubing and a little mini tap if you want to try. You could even drop the tap and just use some really thin tubing.

Last edited by longhairedgit at 02-Dec-2005 01:57