So you want to know how to clean a fish tank with fish in it. That’s the only way an aquarium should be cleaned. With the exception of moving the aquarium, you should never remove fish from their environment. I’m sure, from time to time, something comes along like replacing an undergravel filter or maybe switching aquarium stands or the aquarium itself, that might require removing all the fish but that is going beyond just cleaning the tank. The stress on the fish during normal tank maintenance is bad enough, but using tank nets to catch them and then put them in different water and then move them back after the water change is unnecessary and will probably contribute to higher stress and illness.
Regular maintenance requires a twenty five to thirty percent water change once per month. Some people like to do smaller changes more often and that is ok too. If you have an aquarium that is sooooo dirty that you feel you have to drain it and remove the fish, then you might want to consider the source of the problem. A tank that is not over fed, not next to a window, not used as a nightlight, should not need to have fish removed during routine maintenance. The number of aquarists who leave the hobby because of frustration of tank maintenance and / or dirty fish tanks is way too high. People who own cats have to clean the litter box every day or two. People who have dogs have to pick up the piles in the yard every couple of days. Anyone who has small animals and reptiles knows that certain things have to be done to maintain the health of their pet. Just because fish use gills instead of lungs doesn’t mean they don’t need clean water in order to stay healthy as well. Plecostomus or algae eaters in freshwater aquariums may keep up with algae on the glass or even many decorations, but that doesn’t mean the quality of the water is healthy.