compatibility with goldfish
Previous commentors are correct about the 30G being small for goldfish--my pair grew from 2 inches to 8 plus inches in 2 years, and really deserve a larger tank, but due to lease conditions can't get one unless I move. So we change the filter media 2X-3X/week, 50% water change a week, and have lots of live plants to support water quality; and when spot really gets feisty, she knocks the tank so hard it rocks and knocks scales off herself.
But just on the compatibility question: weather or dojo loaches are very cold-hardy and large enough (8 inches or so when grown) to handle themselves around full grown goldfish. White clouds are cold-hardy but small enough to be lunch as the goldfish get bigger (that happened to one I housed with the goldfish when they were much smaller....he did ok until they got to be about 4 inches long then was lunch one day).
If you keep your tank mid-70s or so, which is supposed to be good for delicate fancies, you have more options but the tankmates must be large enough to hold their own, very fast, or just lucky. My tank stays about 76-82 degrees (no heater, just lots of lights for plants), and I've temporarily put a few warm-water tropical fish in with them; the only ones who have been stuck there for more than a few weeks, and looked like they were thriving, are the original white cloud (at least for the six months or so before he was eaten) and a peacock gudgeon, who has been exiled there for about 4 months now, and is small enough to hide in notches in the driftwood.
