It's a sump...I don't run one but plenty of people here do. They can tell you what you should fill it with...such as ceramic cylinders that allow bacteria to grow. Do you know anyone that keep fish?
In the mean time, try 20% wc a day.
Sent from my iPhone using MonsterAquariaNetwork app
In the mean time, try 20% wc a day.
Sent from my iPhone using MonsterAquariaNetwork app