Hints:
#1
This fish is found in shallow freshwater and brackish lagoons of a certain delta in Romania. It barely reaches over an inch.
#2
This fish's lake in Cameroon gets polluted, which is why it is critically endangered. Adults feed on phytoplankton.
#3
My pride and joy of this round. If anybody made it this far, I'll give you the range so that you can have a chance! Endemic to the Kızılırmak River, Seyhan river drainages, and Sultan Salığı Marsh. If anybody gets this, I will be very surprised!