I have kept and bred all sorts of trops and coldwater fish professionaly over the years ive also kept marine fish and corals, it is an exacting hobby expensive and not for the impatient .
For many years i was Admin/Mod on a forum for marien fish and corals and the mistakes made were mainly overstocking and a misunderstanding of how filtration works, there are many ways of filtering a marine tank and much bad and outdated advice is often given.
mainly start with the biggest tank you can afford ,nano tanks are very unforgiving , 260 litres and no less , you need a supply of RO water you can get from a marien fish shop to mix your salt with , 15 to 20 kilograms of living rock , 2 or 3 powerheads heater lights , lighting depends on what you want to keep , UV unit protien skimmer..shall i go on? lol...
Goldfish are not easy indoors either thier big messy gross feeders, Trops are possibly your best bet, theres a range of micro rasboras and m,any brightly coloured tetras now available to have in small tanks , thier more forgiving over water conditions than marine fish .
If you want to do marine i can guide you step by step on Berlin filtration flow rate stocking etc, the greatest thing you will need is patience and lots of money lol...