My daughter wanted a goldfish a couple of years ago, and me being the flash git went down to the local tropical fish shop and went the whole hog and brought a Jewel fish tank which have a built in-tank filter system which is quite easy to manage, gravel, plants, logs, bubble pump and a fw chemicals to get the tank set up, this cost almost £400 !
The tank needs a couple of weeks to settle with the new water before you can add fish, now this is the time to take a lot of advice from a specialist at the fish shop as they will die if you get it wrong, as said before some of the fish I have put in have died with the slightest issues, temp change, a rise in nitrates etc but some seem to be able to handle anything, unfortunately the hardy fish are a bit dull to look at!
I change 10% of the water every week and the soft filter weekly also, then I change the carbon filter every 6 weeks and the other nitrate filters every 6-8 weeks, never changing them all at the same time as this is where the bacteria needed for the health of the tank live, I add a chemical to remove the chlorine from the tap water every time, I also add aquarium salt every other change to relax the fish and de-stress, make sure you allow this to fully disolve before you add this however as the fish will eat any leftover grains and this kills them (as I found out !)
They can be great fun to look after and look great in the night with the tank lights on, just do your homework and read up on the do's and dont's or it can be an expensive hobby, as for marine tanks you need a lot more cash and attention to get that right and is something for another time I think