wrote in news:1141771061.150422.199360
@z34g2000cwc.googlegroups.com:

aquatic plants and ended up getting 2 Sunset Platies. I asked for a
Male and a Female and it seems like they are (one is skinny-ish and the

Platies are easily sexable by looking at their anal fins. The females'
looks like a regular anal fin, while the males have an anal fin that has
evolved into a stick-like shape that is held close to the body... it
basically serves as a fish penis for transporting sperm directly into the
female.

fine. On a side note: Anyone know how to tell Zebra Danio sex? I am

It's tricky until they're adults, but the males are slim and smaller and
the females are larger and rounder (basically due to the all the extra
organs).

wake up to see what I assume is the male Sunset Platy at the far left
coner of the tank, just skimming the surface and kinda avoiding the
other one. I thought maybe he was nervous but I looked closer and saw
these red markings it didn't have before in the middle of its body

It sounds like it has been attacked, probably overnight. Males generally
harass the females without doing much damage, but they will often injure or
kill other males unless there are too many to single out. It could be your
Chinese algae eater too, as you know they can be aggressive, but their
aggression is usually blatantly obvious and shouldn't be limited to just
the one platy.