How long is it going to take for my fantail goldfish to grow?

i have 2 fantail goldfish in a 10 gallon tank. see what happened was, 9 1/2 years ago i got two baby fantails. i always had them in a 5 gallon tank, and i never really checked water levels and such, they just had a basic filter and were fed every morning and they tripled in size! unfortunately one of them suffered an awful ammonia disease recently, and she ped away. now that i have done more research about all the diseases these little guys can get, ive been taking much better care of her. she is now in a ten gallon tank with a filter, seashells, a small gl ship that's supposed to look like a sunken ship, and 2 small fake plants. (im hoping to get a real one soon). i check the water levels once a week and do a 15% water change once every 3 weeks. i just adopted a new fish and its really little. (about 6cm). i wanted to know how long its going to take for it to grow bigger like my other fish, and i thought maybe it would grow faster then the other considering the new tank and the way i currently care for my fish. any ideas?