What are the organisms that feed on plant sap and other plant parts called?

In an aquarium,two herbivorous species of fish are living together and feeding on phytoplanktons. As per the Gause's Principle,one of the species is to be eliminated in due course of time,but both are surviving well in the aquarium. Give possible reasons.

While explaining interspecific interaction of population,$(+)$ sign is assigned for beneficial interaction,$(-)$ sign is assigned for detrimental interaction and $(0)$ for neutral interaction. Which of the following interactions can be assigned $(+)$ for one species and $(-)$ for another species involved in the interaction?

If the '$+$' sign is used for a beneficial interaction,the '$-$' sign for a detrimental interaction,and the '$0$' sign for a neutral interaction,then which population interaction is represented by '$+$' and '$-$'?

Match the following columns:

Column-$I$	Column-$II$
$P. +,+$	$I. \text{Mutualism}$
$Q. -,-$	$II. \text{Parasitism}$
$R. -,0$	$III. \text{Competition}$
$S. +,0$	$IV. \text{Amensalism}$
$T. +,-$	$V. \text{Commensalism}$
	$VI. \text{Predation}$

While living in and on the host species, the animal parasite has evolved certain adaptations. Describe these adaptations with examples.