(a) Ponds and lakes are natural water bodies. They care naturally cleaned by nature itself using microbes. However, aquarium is an artificial ecosystem and needs human interference for its sustenance.
(b) Ozone layer is getting depleted at the higher levels of the atmosphere because of excessive use of certain chemicals like CFCs (chlorofluorocarbons). These react with ozone and split it into molecular oxygen resulting in ozone hole.
Ozone depletion is harmful because without ozone, the harmful UV rays of the sun will reach the earth and damage the soil, buildings and cause many diseases in humans and animals such as skin cancer etc.