We have had a few goldfish lasting a couple of years each. Not an amazing fish-keeping history
What do we need? I understand they're marine fish and not tropical so it's salt water. Will it be expensive to set up the tank? Can I just have one little fish swimming about on its own or will he need some pals?
Also, I had a quick look in the pet shop this morning and what was labelled clownfish was miniscule and looked nothing like Nemo. Do they grow into their appearance?
Marine tanks are WAY more hard work, tricky and expensive, and while I think you could have just one in a tank, I think you would need to have coral etc in there for it to live on/in, but don't quote me on that bit.
But seriously - do some research on what is involved in maintaining a marine tank before you decide. It is a whole different kettle of fish to tropical. One place to start I have tropical, and think marine tanks are amazing, but a) can't afford one, b) am not not prepared to spend my life monitoring water parameters.
In a nutshell, yes they are difficult and yes, it will be very expensive to set a tank up.
Could you talk her into some easier fish and set up a little tropical freshwater tank for her? Go to a decent aquarium stockist (like a big garden centre with fish outlet) and ask for advice about a starter kit for a tropical tank.
Only thing is though, clown loach can grow to 6-7" and you need a group of them so you'd need a bigger tank than you probably want for your dd. I have a pair, about 4 years old now in a 220l tank. I'd look at something like some of the more robust tetras or a siamese fighting fish (very attractive) that would be happier in a small set up and are fairy hardy.