Get 12 fish bags from your local aquatics store. Bag the goldfish individually and the tetras in groups. Make sure you trap loads of air in the bags (more air than water) to give the fish plenty of oxygen and make the bags very, very firm. Tie a tight knot in the top, invert the bags and put them knot first into a second bag. Tie this off as well. This keeps the bags strong and helps prevent the tetras being caught in the corners of the first bag.
Prepare a box big enough for all the bags and fill with insulation materials such as layers of scrunched newspaper and polystyrene. You need to keep the tropical fish warm and all the fish cusioned. Pack the bags into the box, making sure they can't rattle around and are fully cusioned. Be prepared to use lots of newpaper!
Take down the tank - empty all the water and gravel. If you have a sealed water carrier, take as much of the water as you an with you. Pck the gravel and any live plants away into a bucket. If you can get more bags and totally encase the plants in water, that is best.
You MUST keep the filter in water, or at least the filter media (the sponges) or you'll kill the bacteria that are keeping your tank alive.
You need to empty the tank as otherwise, you could get uneven pressure from the sloshing water and gravel which would strain the glass.
When you get to yur destination, set the tank back up as soon as you can, top up with fresh, dechlorinated water and carefully re-acclimatise the fish (as you would when getting them from the fish shop) and get the filter working ASAP. Ta dah!
As a point of interest, you do know that goldfish are not tropical fish, right? Fancy/fantail goldfish do prefer temperate conditions (so sub-tropical) but common goldfish need cold, high oxygen water. Not keeping them in the correct conditions makes them more prone to disease and will shorten their lifespans considerably. These are also very large fish who should reach 12-18" as adults and will outgrow your 55g. They should really be kept in a pond.
Keeping any goldfish with tropicals, even fancy goldfish, is also risky. Goldfish require massively over filtered water and a higher water flow than most fish. They also require cooler temps than tropicals and grow big enough to inadvertently consume or kill smaller tropical fish. They can be quite physical in their interactions!
A 55g would be perfect for 4 full grown fantails and would look stunning with those 4 brightly coloured, 8 inch long fish. Plus it could be kept a room temp so you'd save energy.
EDIT - just saw that you are moving them to a new tank. To cope with the goldfish the tank needs to be as big, if not bigger, than your current one and when then standard goldfish reach 8-9 inches they NEED to be moved to a pond as they could reach 18 inches and need far more space than could be practically provided by an indoor tank.
