Planarians are nocturnal. There are two common types of planarians in aquariums: one is commonly called round-headed planarian, and the other is commonly called pointed-headed planarian. Besides these two basic categories, planarians can actually be further subdivided into many species.
Planarians found in aquariums are usually introduced by newly purchased aquatic organisms or aquatic plants. Planarians cannot survive in severely polluted water. When there is too much organic matter in the aquarium, causing the water quality to reach a level of mild pollution, you may see planarians appearing on the aquarium walls after turning off the lights at night.
The planarians in the middle and right of the above image are the forms that may be seen in aquariums during the initial tank setup period (first six months to one year). They are about 2 ~ 5 mm in size, very small. At this stage, planarians look like short, plump white lines that move, and you may see them dividing in two (the rightmost image in the above picture).
