I have a baby goldfish that is all alone in a fully cycled 10G tank. Is there any other fish(es) I can put in there so that he's not lonely. Preferably something other than a goldfish. Thanks for your advice.
Sorry but I would say probably not. How about a different "type" of goldfish? Fancy with long fins or those ones at the LFS that look like their eyes are bulging out? I have 3 goldfish and have tried other fish and they all die when they were with the goldfish. Part of the reason is goldfish like is cooler and don't mix with tropicals that like warmer temps. Especially a 10 gallon doesn't give you much space to work with. I too have my goldfish in a 10 and am looking for a 20+ gallon replacement tank. I would probably only get 1 more goldfish unless you plan to get a bigger tank soon. My goldfish are definetly begging me for a larger tank right now.
Usually the recommendation is 20Gallon for the first goldfish and 10 Gallon for each goldfish you add. so I would say the one goldfish in the 10G. is all you can have. The reason for this is that godlfish are heavy waste producers. HTH