* Different Needs: Sea urchins are saltwater creatures, while guppies are freshwater fish. This means they require drastically different water conditions (salinity, temperature, etc.) that are incompatible.
* Space: Sea urchins require more space to roam and explore than a 10-gallon tank can provide. Guppies, while smaller, also need adequate space to swim and avoid overcrowding.
* Feeding: Sea urchins are herbivores and eat algae, while guppies are omnivores. It's difficult to provide both with their appropriate diet in a single tank.
* Compatibility: While sea urchins may not actively attack guppies, their spines can cause injury, especially if the tank is small and crowded.
Conclusion: Combining sea urchins and guppies in a 10-gallon tank is not feasible due to their different needs and potential risks. You should choose either a saltwater setup for your sea urchin or a freshwater setup for your guppies to ensure their well-being.