I asked the guy in the pet shop what I should do, other than buy plastic plants to decorate their tank and he recommended putting slices of zucchini in the tank.
So next visit to the supermarket I purchased a zucchini for this purpose and into the tank went what I thought was a nice piece for the fish.
By the next day the water was putrid. The smell was awful and the water went a white cloudy colour. I had to change the water completely, clean out the tank and replace a goldfish.
The next time I visited an organic grocery store, I purchased a zucchini and put a slice into the tank. Everything was fine. Goldfish enjoyed water remained clear and habitable.
I thought about the first repeating the zucchini experiment because I thought perhaps I just got a bad zucchini before damning all zucchini from the supermarket to hell based on one experiment was a bad idea. So I purchased another one from the same supermarket and into the water went a slice and exactly the same deal, the next day the water was putrid again and had to be replaced.
I wondered whether I was just lucky the first time with the organic store and again I repeated that experiment but the water remained fine again after giving the goldfish a piece of zucchini.
Again, this was years ago in Australia.