do a lot of different things to show u can be responsible
I have many fish, a guinea pig and a hamster (got my hamster yesterday evening). But the fish are ok. The have bred and we have many baby fish.
What type of fish were you looking for? I have tropical, which I read aren't as hard to look after as salt fish. If you really want them, you need to consider which type, what size tank, if you have money for filters and running costs. Maybe just try looking after a goldfish, then if it works go onto tropical.
As for persuading your parents to let you have one, maybe try searching on google under 'ways to convince my parents to let me have a fish'.
But I had to persuade my parents to let me have a hamster. I said I would look after him and that I would pay. Also I said he would be in my room. But if you have a cat, then make sure that your fish are safe.
Show your parents that you are responsible by doing chores and looking after your guinea pig and cat. Maybe feed them for a week or so and then ask again.
Draw up a plan of where you would put the tank and what you would do. Be very organised.
Ask them, but don't be too annoying.
But things to take into consideration...
Do you have enough money to pay for them (even if you earnt more)
Do you really want the responsibility of having to take care of them
Are they really what you want
Ways to earn money would include:
Doing extra chores
Maybe sell some of your old things
Help around the house
Maybe help an older relative
Hope I helped and hope you get your fish.