A SINGLE fancy goldfish such as a bubbleye will just about fit in a 30 gallon long ( not tall ) when fully grown. But no more than that. Two will be utterly cramped when fully grown so unless you have a bigger tank ( they grow surprisingly quickly in their first year or so ) then don't attempt to keep more than one . You must always take into consideration the ADULT size of the fish before getting one. When you first buy the fish it'll be a weeny little thing who might look a bit lost in a 30 gal. But with good feeding and regular weekly water changes it WILL grow so don't be tempted to get another goldfish to keep it company. They do perfectly well alone.
With a single fancy goldfish in that tank you could have some smaller tankmates such as around 8 white cloud mountain minnows. But apart from that there really isn't much you can put in without running the risk of losing swimming space when the goldfish is fully grown. Most fancies get to be about as big as an orange and despite being weaker swimmers than normal types of goldfish, they are still active fish nonetheless and need space to swim and grow.
Don't forget, yu will need a large cannister filter to cope wih the waste of a goldfish. An external filter that can hold plenty of media, like an Eheim 2213 for example, would be ideal ( you do not have to buy one brand new, look out for second hand ones on ebay or in the classifed section of newspapers ect ) .
