How to match temps when doing large water changes?

I've never really worried about temp differences personally, unless the temps are extremely different... in which case, i would normally make sure that the heater i want to use is under the water surface, and then just extremely slowly fill the tank back up, allowing the heater to somewhat heat up the water temp as it goes in...

Other methods i've heard of, is to get a large drum, and fill it with water from your tap, then place a heater in there to warm the temp up, during this time, people also add ferts / prime etc to the mix, and then using a submersible pump, pump the water from the drum into their tanks...

Use warmer water out of the tap.

I almost killed all my fish in my 90 gallon when I was refilling with cold water. I didn't realize how much the temperature had changed and noticed the fish were all going into shock and laying on the bottom. I have a long python hose so I ran upstairs and used hot water from the tap. I don't have an attachment for the sink so I had to hold the part that usually goes into the tank under the faucet. I also usually boil some water and mix it with the cold water as I refill the tank. This works well if it is not too cold.

Yeah I always monitor the temp when I refill via a python. I start in the middle (single handle) then let it hit my wrist to judge the temp like you would a bottle for a baby. And I check my glass floating temp gauge during the refill as well.

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I usually just do my best to keep the water from the tap (via siphon) at only a little colder than the tank water.

I use the thermometer in my tank to check the temperature coming out of the tap, match the temperature and then fill! Seems to work really well and all my fish seem pretty happy

After four some years I can tell each tank by testing temps by hand, basically just by feel. But you can try this; I stick tape at the bathtub temperature appropriate for each of my 3 tanks. Takes a while to get this right but eh, worth it.

I usually refill with colder water but not too cold. I heard it can help stimulate breeding in some cases.

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I know there are a number of aquarists who only use the cold water in water changes, not trusting the hot water for whatever reason. This has always seemed risky to me, as the tank temperature with a 50% change would lower significantly, at least for me as my cold water is usually far too cold to keep my hand in it.

I mix hot and cold at the faucet using my hand, and like a couple of other members suggested, in most cases I deliberately have it a tad cooler. The only time I haven't done this is with tanks housing very sensitive fish like Chocolate Gourami, though I admit this may have been unnecessary even here. The cooler water does stimulate fish, as it replicates a rain in their habitats, as rain water is cooler than the surface water the fish are in. But this cooling is in my opinion not as drastic naturally as it would be running very cold tapwater into an enclosed aquarium. The mix of cooler rain water in the habitat would be much more gradual.

Difficult to spawn fish species can often be stimulated into spawning by water changes with cooler water. And especially if done on a day of low pressure, something the fish can detect.

Byron.