What are these tiny creatures swimming around my aquarium?
Hard to tell because of the poor picture/video quality, but almost immediately the body shape, distinct &quot;face&quot;, size and behavior made me think of a small invertebrate animal called a rotifer (Phylum Rotifera).

 Rotifer eating protists. Photo Credit: Jacqueline Ronson, 2016 
There are about 2000 species occurring worldwide within this primarily freshwater phylum, each of which is fairly simple anatomically. Their name (which means &quot;wheel bearer&quot;) comes from the two crowns of quick-moving cilia they possess on their heads:

 Source: Gifer 
These cilia attempt to push food posteriorly toward the rotifer's back-set &quot;mouth&quot; (which is actually a modified pharynx called a mastax:

 Photo Credit: Jean-Marie Cavanihac 
Rotifers are small (always &lt; 2 mm) and so eat small planktonic and protist prey. From UC Berkeley:

As rotifers are microscopic animals, their diet must consist of matter small enough to fit through their tiny mouths during filter feeding. Rotifers are primarily omnivorous, but some species have been known to be cannibalistic. The diet of rotifers most commonly consists of dead or decomposing organic materials, as well as unicellular algae and other phytoplankton that are primary producers in aquatic communities.

As for body shape:
Although all Rotifer species are bilateral animals, they vary quite a bit in body shape. Box-like shaped rotifers are referred to as being loricate, while more flexible worm-shaped species are considered to be illoricate.

 Photo Credit: Juan Carlos Fonseca Mata 
Regarding sometimes staying &quot;rooted at the tail&quot; as the OP observed:
Some rotifers are found usually free-swimming, while others &quot;glue&quot; down their posterior &quot;toes&quot; to anchor themselves to some substrate or debris. Again from UC Berkeley:

The final region of the rotifer body is the foot; this foot ends in a &quot;toe&quot; containing a cement gland with which the rotifer may attach itself to objects in the water and sift food at its leisure.


 Source: Starr et al. 2013 (Cengage Learning) 
See here for further &quot;fun facts!&quot;