Dinobryon

Flagellated protozoa Dinobryon (not harmful)

Leeches - Theromyzon tessulatum

Fresh water leeches are parasitic. They will suck onto a fish and feed on blood. On those two photos above one can clearly see young leeches developing inside the big one. Leeches are viviparous, they give birth to fully formed small leeches.

Photos by Dusko Bojic.

Cyclops - copepod

This microscopic crustaceans is well known as the Cyclops. The photo presents an egg carrying female. This copepod plays a very important role in the underwater food chain. Not only small fish feeds on them, but huge Whales also. They can be found in both, salt and fresh waters.

Photo by Dusko Bojic.

Ostracod

Ostracod is a microscopic crustacean (0.1mm-3mm). These in the photo above are no bigger than 1mm. They feed on dead plant matter. Fish will readily devour them.

Photo by Dusko.

Stentor protozoa

One of the largest protozoa, Stentor, is a unicellular, trumpet like, protoplazmatic-microorganism. It feeds on bacterias and smaller protozoas like Rotifers and Water-fleas. It can grow to about 2mm.

Not sure, but I believe that Shrimps will eat Stentors.

Loxophyllum ciliate

Ciliates are unicellular protists that use hairlike cilia for feeding and moving. The largest Ciliates can reach 0.3mm like the one in this photo above. They feed on bacteria. Larger microscopic life-form, shrimps and snails will devour Ciliates.
Photo by Dusko Bojic.

Oligochaete worms

Oligochaete Annelida spp.Annelida performing a reproductive form 'paratomical fission' where a new head is formed in the middle of the organism and then break away from the parent worm leaving it with a new tail and the other Annelida will swim away.

Oligochaete Chaetochaster diastrophus
Commonly found in ponds and aquariums. The ones in the photos belong to the fresh water habitats. They are harmless. Fish will readily eat them. They can grow to about 4 mm. Their bodies are very transparent which exposes their well developed digestive and nervous system.

Photos by Dusko Bojic

Flatworm

Phylum Platyhelminthes
This worm can grow to 15 mm and in most cases is harmful. The one common in aquariums and ponds is the Turbellarian which feed on microorganisms. There is over 25.000 species of this worm and many are free-living like this one on the photo, but many are also parasitic.
Fish will eat them.

Photo by Dusko Bojic.