I can't see what filtration you have on that 8 gallon, but as long as you have a power filter appropriate for that size aquarium you should be good. I wouldn't rely on bubbler types.
There will be lots of variables contributing to your shubunkin's growth. Like most goldfish, they can grow extremely fast if the conditions are ideal, though I think shubunkins grow just a little slower that plain old goldfish. I don't know where in the country you are, but you are likely wanting to put these little guys in the pond in the spring, right? That should be doable with the right conditions.
To get the fastest growth from them:
Keep water very clean and well oxygenated and change some of the water regularly to avoid growth inhibiting hormones from building up.
Feed an appropriate amount of good quality goldfish food (I like mini pellets for small goldfish)
Keep water temperature at a slightly cool room temperature - too warm or too cold slows growth. Goldfish can survive VERY low temperatures but they don't grow at all when it is cold - if it is as warm as for tropicals then you can have problems with disease.
Just remember that even a fully adult, ten inch shubunkin can still fall prey to raccoons and cats - give them some deep water once they are outside! Good luck and keep us posted!
