Yes jotto, not a silly question at all - its what we frequently refer to as "good water changing technique" and is an important skill for beginners (on a big hobbyist forum you often get lots of old experienced aquarists yakking about various loose ways they change water but my opinion is that they are so experienced at it that they've forgotten that they ever had to learn the basics in the beginning, lol.)
The temperature part can be described as important but not so critial as to be a hassle. You don't need to do long comparisons with thermometers. What I recommend is that you become practiced with using your hand to judge. I always use a big plasic cup filled with some of the old tank water that came out into the catch-bucket during the out-siphoning. In my own case I have a fiberglass lined hot water tank and am not worried about excessive copper or other metals (as a subset of our UK people are, from what I gather) so I stand at the sink putting my hand in the cup and adjusting the faucet to something that matches. When it feels about the same that means I'm well within the temperature tolerances of the fish. (In my case, my faucet lever retains this temp setting and I can turn it off and hook up the hose to this faucet head for direct filling of the aquarium.) The reason I advocate that beginners temperature-match by habit is that this is the safest recommendation that will cover all situations and often (as in your case) large water changes will be needed, so too much cold or hot would be too much of a shock to the fish (there will be plenty of time in the future to learn about -desired- colder water shots that stimulate breeding and special things like that - for now, as a beginner, its much better to be safe than sorry.)
Another "re-fill" part of good technique is to "condition" the water. This is simply the use of a product to remove the chlorine or chloramine that your water authority puts in to keep down the bacterial count in the pipes so humans won't get sick. I've almost never known a beginner not to come away from the LFS without a bottle of this. The one tweak to this that I recommend is to dose this stuff at 1.5x to 2x whatever the instructions say but not more than 2x. This is specifically for people who are cycling and/or have tanks building up to the first year of filter maturity (beyond that the extra dosing is not needed.) The reason is to gaurd against that rare occurance where the water authority overdoses the chorine product (yes, they do sometimes "shock" their pipes with extra chlorine) at the same time as you, the cycler, happen to have a young, weak colony of new bacteria! If you don't have, or are replacing, conditioner, I recommend Seachem Prime as the stuff that gets glowing reports from nearly all our experienced people. Its got great extra features for beginner problems.
Finally, back to the business of taking out the water in the first place! Always do that by doing a "substrate-cleaning" process (us a gravel cleaning siphon head if you have gravel, or a sand-technique if you have sand.) Why the heck would I do this in a new obviously clean looking tank, the beginner will ask? The reason is... invisible! The nitrite and nitrate products we are worried about are not visible, obviously, and they are mostly charged ions. They tend to be attracted to other molecules that happen to have opposite charge and often those are larger organic molecules that have enough weight or size to get themselves collected down in the gravel where the circulation has been hampered a bit (again, all of this still being potentially invisible to you, the aquarist!) Now nitrites(NO2) and nitrates(NO3) are things you truly want to go out of the tank as much as possible during your water change, so you want to get your clear, cleaning cylinder (or your simple hose if you have sand) down there where they are hanging out! Its more work than simple "bailing" but its something you must always do for the most part and a really good solid habit to establish for yourself.
~~waterdrop~~ (
congrats! you won the coffee induced morning excessive answer, lol)
lol
