Hi Guys

I have four fantail goldfish in a 40Gal tank and was just wondering how much water I should be removing each week during my water changes. I know they should be in a bigger tank but when I bought this house the home owner here left 12 in a 12 gal tank. I didnt know alot about fish but I knew that was way to small. We were never able to get a hold of him to come and get the fish because we didn't know how to care for them.

Long story short, we were able to get the bigger 40gal for the time being and I started to try to care for them. 8 of them passed away over the years but we have had these four going strong. I just want to make sure I am changing enough water.

Also can real plants be planted in a goldfish tank? Like I was saying guys I am just trying to learn as I go along as I was kind of thrown into the hobby. I forgot to mention no local stores would take the fish and no luck on the ad listing sites. So Im basically adopting the little fellas ( actually there getting bigger )