Any Idea Plz Help 40 Gallon Goldfish Tank

I have 40 gallon tank with 4 goldfish........I have installed filter but the problem is water gets polluted fast.....I change 30 % water in a week....I have heard a carbin filter will make water crystal clear......is carbon safe or it will finish the beneficial bacteria from aquarium......what should I do.....I am confused......

 

40g is pretty small for 4 goldfish... And 30% of water change isn't that much. Since you are overstocked you'll have to do water changes frequently anyway. If you want less water changes to do, you'll need a bigger aquarium:/

 

What type of goldfish do you have? How large are they? What type of filter do you use?

 

I would do 3 Fancies at most in a 40g. I would also do 20% WC every other day

 

Carbon will do well to take particles out of water, but overstocking a tank will cause the particles (among other things). Fancy goldfish should have about 20 gallons each. The fish in your tank have about ten each, I'd really recommend that you rehome two of the goldies (I could possibly take them in, we're setting up a huge system in the back of the house).