overcrowded Goldfish aq

Question
i am new to fish keeping and have 20 gallons tank
with 4 goldfish and 2 blackmoors in it and they are doing fine . they are healthy . and i change the water including the gravel cleansing once a week . is it ok or not

Answer
Hello Wajid:  The tank is over-crowded.  The rule to follow with goldfish including black moor fish is one goldfish per ten gallons of water.  You have enough goldfish for a 60 gallon tank.  There is some room to adjust within this very basic goldfish rule... so the trick to keeping your fish healthy is to do water changes twice per week and a gravel cleaning once per week.  Add beneficial bacteria during each water change and invest in a freshwater test kit that will help you to monitor the water chemistry of the tank.  Goldfish give off more ammonia then most fish and this is the reason they need so much water per fish.  To keep them healthy in a tank that is crowded means that the fish keeper has to increase the tank maintenance to meet the water requirements of the fish.  If you need additional help let me know.  dave