Hi, I have moved into a house and inherited a garden pond, so forgive me, because I know very little about ponds and fish. I've noticed one of the fish (looks a bit like a goldfish, but it's black) looks like he's swallowed a tennis ball (see the photo attached). I don't think it is pregnant because I know it has been like this for about a year. What's wrong with it... and is there anything I can do to bring it back to normality? Also, I could do with some advice regarding pond plants. I'd like nice, but cheap (I'm on a very tight budget). What would you suggest?