I'm confused! If it's only a 20 gallon tank, why not use an old school, no fuss no muss cheap syphon that you use by hand, to empty into buckets, then refill using buckets? If you have small fish that are likely to get sucked up (or as in my case, tiny fish, tiny fry and tiny shrimplets that WILL get sucked up) then slow and heavier work is better than hooking up hoses.
A 10-15 litre bucket means you can refill fairly quickly, after temperature matching the new water to the tank temp and using declorinator, and then you can control the flow of adding water back into the tank gently using the bucket and a jug.
Don't forget that if you must use water that's much colder than the tank temp, that you don't have to refill the tank to the top at once. Could replace half of the water you've removed, wait a few hours for that to come to temp, then replace the rest of the water.
As for cleaning the substrate without sucking anything up; as
@Essjay said - carefully!
I always have my thumb over the end of the hose that's going into the bucket so I can pause the flow in an instant. Not only does that mean you can prevent sucking up anything you don't want to suck up, it also means you can pause the flow when you're moving the other end to the next spot you want to clean, so you don't have the tank half drained before you're even halfway through.
You could also try a fish tank divider - not one to install permanently or properly, but to gently shoo the fish to one half of the tank, prop divider in there (this is easier if you have a lot of plants that will hold it up) then work on the half of the tank the fish aren't on. The odd fish will likely go around, but it should keep most of them out of the way while you work. Then you clean one half of the tank on one water change, switch and do the other side next time.
Don't beat yourself up! You'll pick up tips and tricks along the way, and find the methods that work for you! We all had to learn how to do this, we were all new to this once, and your methods are gonna vary person to person depending on what you prefer, what your tank set up is, where your closest sink is! etc. You'll get the hang of things, try not to be so hard on yourself.