It is not recommended to keep sea urchins and guppies together in a 10-gallon tank.
Reasons
- Different needs: Sea urchins are saltwater; guppies are freshwater.
- Space: Both need more room than a 10-gallon tank can comfortably provide.
- Feeding: Urchins eat algae; guppies are omnivores.
- Compatibility: Urchin spines can injure guppies in a small tank.
Choose either a saltwater setup for a sea urchin or a freshwater setup for guppies.