Complex sentence: In addition to the centre tank and habitat tanks the oceanarium has a large number of theme-tanks with fish or other marine animals, which may contain animals which are impossible to show in other tanks, or animals subject to special theme exhibitions.
Split into simpler sentences: In addition to the centre tank and habitat tanks the oceanarium has a large number of theme-tanks with fish or other marine animals. They may be animals which are impossible to show in other tanks, or animals subject to special theme exhibitions.