The most common way for goldfish diseases to spread from tank to tank is through the use of the same equipment in 2 or more aquariums. The only surefire method of prevent cross contamination is to have a separate set of equipment for each tank. However this can be expensive and impractical, so it is important to sterilise equipment properly.
The best method is to use two containers: one for disinfecting, the other for rinsing.
The best active ingredient to use in the disinfecting container is formaldehyde, which is available from fish and aquarium shops, and online.
The normal concentration of shop bought aquarium formaldehyde is 37%. Adding 2 tablespoons of this to a medium-sized bucket will be enough to kill bacteria and parasites.
Simply dip the net, scraper or whatever into the formaldehyde solution, then into the rinsing container which contains nothing but clean water. There is no harm in storing the equipment in the disinfecting container either.
A good tip is to add a small amount of dye to the disinfecting container. Not only will this easily distinguish the otherwise colourless disinfectant from the rinsing water, it will also give an indication as to when the rinsing water needs changed as it gradually gets the colour of the dye.