Even if you do 100% water changes, as long as the water is de-chlorinated (which it should be for the sake of the fish...), there will be some beneficial bacteria in the gravel (provided you have gravel
) so you can still use this to speed up the cycle.
Zenn, no offense but you REALY need to do some research.
'Goldfish' is NOT a general name for all coldwater pond fish! Goldfish (include, common, veiltale, black moore, bubbleeye, celestial, fan-tail, pom pom, ranchu, ryukin, wakin and all other fancy selectively bred varieties) are carassius auratus. They are NOT the same as koi etc... Koi are cyprinnus carpio, darters and those other random coldwater fish you mentioned are NOT goldfish.
You said that larger fish have bigger brains therefor are smarter and also said that fish don't 'remember'. My platies (as an example) come to the surface when I feed flake (obviously). The next day, they will go to the surface again when they see me approaching. However, if I feed them sinking pellets, they have to go to the bottom to eat - the next day when I approach, they go to the bottom - now that isn't just instinct - they actualy remmber what I did the day before. Notice that platies are smaller than goldfish.
As for size - ALL goldfish grow to AT LEAST 6". True, some of the more selectively bred VARIETIES of carassius auratus are smaller but NONE of them remain at 2". Only a pair of 2" goldfish could be kept in a 5 gallon - and those simply do NOT exist! Even then, this would apply to tropicals. Goldfish are MUCH messier and produce a lot more waste so require a larger tank to dilute the ammonia.
The 'carnival' goldfish you mentioned are typically commons. Sometimes they are commets or sarassa comets and occasionaly you'll get a nice shubunkin - these all grow to 12". Fantails also get to at least 8" and veiltails typicaly to 10".
NO goldfish is suited to a 5 gallon tank! SOME varieties can be kept in a 10 gallon but most need at least a 20 gallon with 10 gallons for each additional goldfish.
The fact that they are very messy and social as well means they require a bigger tank still if they are to thrive.
Now I know this discussion is over but you realy need to get your facts straight - I suggest you do a google search on goldfish and try to find some good sites - I suggest this for a start:
http/www.bristol-aquarists.org.uk/