A comet will do the bulk of it's growth in the first two years (where the growing is obvious), and then slow down after. Of course this will depend on water quality & food.
That being said, goldfish are dirty as heck.
I have 4 in a 75g tank running an Eheim Pro 3e 2078, and one male BN pleco. Mine are 'fancies' though - 3 fantails & 1 ryukin; they are 8.5 yrs old, 6 yrs, 6 yrs, and 3 months old. Lisa, who is the oldest fantail is 7.25 inches from nose to peduncle (that's right before the back fin) and 365 grams.
I do a 40% to 60% WC every 3 to 5 days and test the water parameters like a crazy woman. The tank is barebottom other than large rocks (they forage so much that substrate became a choking hazard), fake plants (they eat every live plant I've tried!) and driftwood for the BN pleco.
I've had platys, mollies, discus, tetras, clown loaches, corydoras, apple snails, mystery snails, ghost shrimp (just to name a few) through the 14 years I've been keeping fish, and when it came time to tone it down to one tank, the goldfish were who I kept
Nothing wrong with keeping goldfish at all
