get a bucket and take out the water you want to change and you dont need to take any of your fish out. just leave them alone in there and just get a big enough bucket that could fit in your tank and take the water out. take the water out, much as you prefer to take out. and now when you are done just put your finger in the tank water to feel the temp ( you have to be good at this ). and now go fill the bucket with new water making same temp as in your fish tank. you might have to turn some warm and some cold water on utill you get the same or similar temp as your fish tank ( i do this from my bathroom shower tap). its very easy and simple and dont worry you wont kill any fish by doing this. but becarefull not to let any soap or liquid go in the water bucket. just fill a water with bucket and add your water conditioners. and put that water in your fish tank after like 1 minute or so. keep repeating this till you fill your tank. but remember its very important to keep the water temp same or similar as your tank water. its ok if its off by 1-2F. now this is the easiest way to change the water and will take you less than 10-15 mins to change the water.