Cigar

The tank is about 120L rectangular shape, the previous owner has had them in there for a while

Help with any of my questions would be great thanks. Just brought a fishtank (120L) 3 days ago that came with 10 coldwater goldfish the owner said it could probably take about 10 more, is this right or too many for the size of the tank? How much and how often should i replace water in the tank? Currently feeding floating pellets once a night (enough to last 10 mins) is this all the fish need to eat? Do i need to use the light at the top of the tank daily for a few hours? How often should i change the filters in the tank (currently got an under gravel filter that pumps water up through two tubes to the filter stones)? Is there somewhere i can send/post pictures of my fish to find out what sort/breed they are?

Hi to all who read this, I've recently purchased my first fishtank included with 10 goldfish inside, and am enjoying learning some of the many things there are to know about keeping goldfish. I hope to learn from many of you who have kept goldfish some hints, tips and maybe even some secrets on how to keep mine and the goldfish' experience the best possible. Bye for now Craig