09-02-2013, 08:48 AM
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That's definitely a female guppy, females are actually bigger than males, and also can have bright colors like males though it's not as common.
Guppies are usually fine in a group of five in ten gallon tanks, but it is pushing it. They will need a lot of hiding places or they'll be nippy. So more plants will help, I'd also suggest a small cave. Buying mature females, or ones over five months old, you also have a high probability of pregnant females. They can get snappy when pregnant. Sometimes you also just get a problem fish, one that isn't happy no matter what. Those need to be removed.
Did you cycle the tank before adding in the fish, you said you bought them THEN set it up....having a fish-in cycle is dangerous and often fatal for fish. The spikes can also cause some nasty aggression since they're uncomfortable. Be sure to check your water stats, make sure there are no spikes. No ammonia is a "safe" amount, same with nitrites. Nitrates can be handled in small amounts under 20ppm, but they can also cause aggression in your fish because it makes them uncomfortable.