50g tank needs suggestions.

[jkcloud1]

What would be a good fish to put in with these?

 

[V227]

Maybe a few Synodontis catfish. There are numerous varieties available. They usually are very active swimmers and make a good clean up crew.

 

[Toad]

None, that tank is stocked at the very least. Common goldfish get big, and should really be in a pond.