Here's why:
* Goldfish grow large: Black Moors can reach up to 12 inches in length. A 20-gallon tank is simply not big enough to accommodate a goldfish of that size.
* They produce a lot of waste: Goldfish are messy eaters and produce a lot of waste, which requires a larger tank to be properly filtered and cleaned.
* They need space to swim: Goldfish are active swimmers and need ample space to move around. A 20-gallon tank would be too cramped for a Black Moor.
A more appropriate tank size for a single Black Moor would be at least 55 gallons.
For multiple Black Moors, you'd need an even larger tank.
Remember: A larger tank is always better for goldfish, providing them with a more comfortable and healthier environment.