Hi, I have a 450L tank with 13 small to medium goldfish{ the biggest is 15 cm}. there are 2 external filters doing around 3200 L/H. Water P are 0,0,20 and were doing wc of 30% once a week. im feeding with JBL Novo Pearl.

all of the goldfish are rescued and some of them i have for around 3 years. it seems like when i bring a small new guy he often grows but then stops in a relative smal size. the 15 cm one i have for 3 years and he hasnt grow in the last 2 year..

what do you think is the reason? is it possible its some kind of health issue?