Moorfish in corner of tank. Help!

[JessicaWilson]

About a month ago I got a fan tail and moorfish goldfish in a 20 litre tank.

The moorfish has been fine but recently has been staying in bottom corner of the fish tank. If I walk in front of the tank he swims around but when I walk away he is back to the corner.

Is he sick? The filter is too strong? I'm a new fish owner so this behaviour is alien to me. Help!

 

[Sunnyfish]

i don't know all that much about goldfish, but can't moors and fantails reach up to 6 inches or more? i changed 20 liters to gallons since that's what i'm used to and it translates to 5 gallons, which sounds far too small. maybe that's the issue?