I currently have 4 goldfish, all about 2-3inches and I am about up upgrade for long term (so I wont have to upgrade again),

I have been told for 4 goldfish I will need a 300L fish tank and that will do me for life, theyll be okay in that until they pass,

Is this true or am I going to be needing to upgrade bigger than 300L eventually?

Also, when I go to my pet store I keep seeing this Black Moor that has a cool white edging colour to his back fin (looks healthy, just the colour) and a bright orange tummy, if I got him, would I THEN need bigger than a 300L or would 5 goldfish in a 300L be fine?

please answer and potentially suggest fish tanks (send links)

This article explains how to work out what size tank you need. Every additional fish increases the size you need to house them. 300 litres will be OK for what you already have, for life, but if you add more fish you'll need a bigger tank.

http://www.fishkeeping.co.uk/articles ... -size-life-expectancy.htm