Most suggest using old tank water but I use clean water that has been treated with Seachem Prime water conditioner. As long as the water has been treated to remove chlorine it is okay to use for cleaning. If you are using the loose filter floss just change it when it gets really dirty. I have not tried to clean that type of filter media yet. I did notice a change in ammonia level when I did the first cleaning of my new small canister filter. I had a small ammonia spike even though I didn't take out any bio rings. I switched to a sponge after that and just clean it. All other filter pads can be given a good beating in a bucket of treated water. I sometimes take out my bio rings in the HOB
and give them a swish or two in treated water. My tank builds up a lot of gunk. For water changes, I just add the Prime to the buckets of water and then slowly pour it into the tank with a small bucket. I actually bought a children's sand pail to scoop water out. I do up to a 50% water change each week because I use dry fertilizers and have a lot of molly fry in the tank right now.
Be sure to add new stock slowly, a few at a time, so the bacteria can grow to take on the new ammonia load.
I thought I would add that when you get another tank, as we all end up doing, You can pour the cleaning water into the new filter and cycle it in record time.