If you mean a partial water change you leave the fish in. Just syphon around them. They normally move out of the way but be careful not to knock them or you might damage their coating. If you mean to completely clean the tank to start again they would have to come out. If you want to clean algae off with a scraper or rearrange plants they still stay in. I'm talking about ordinary fish, if you have big ones that bite I don't know about that.
I have a gravel cleaner which you plunge several times to start the siphon off, and then i hoover round everything till about 20% of the water is in the bucket. I then replace it with dechlorinated water of about the same temp as the tank.
so could i put a pipe in, drain some of the water and then add some water in? i use safe guard that i got from pets at home should i use something else?
Big Als Online also has some dechlorinator that I use, it's their all purpose stuff. I find that it works fine. Also if you have a big tank I'd suggest getting a python that hooks up to the sink so you can empty it and fill it up quicker without buckets. I use it on my 55 gallon and it makes a difference for me anyway because I make a mess with buckets.