Finding the right sized tubing

[scubaethan]

I have a 12g nano cube and upgraded from the stock pump to a maxi-jet pro 900 last night. I guess the added power is too much because now water is spraying backwards around the tube that connects to the hole in the plastic wall for the return. I feel like this is a really dumb question, but how to I find the perfect sized tubing to fit that hole?
Thanks