Unusual Guppy Behaviour.

[hepahtitusd]

So in one of my tanks, 4 gallon with two guppies, one of the guppies has suddenly started to be highly aggressive toward the other. They have lived in the tank for 6 months and always gotten along really well, but all of a sudden this morning I noticed my blue guppy hiding and whenever he comes out the red one immediately attacks him (they are not playing I know what it looks like when they usually play). Other than that there doesn't seem to be anything thats out of the ordinary with them. Anyone have any ideas on why he would just turn on him?

 

[thedayawaits]

We had a situation like that where one female guppy suddenly became extremely aggressive to all the other fish in the tank and would chase and nip them. She had grown from fry and had just become a mid-sized fish. I wonder if it had to do with a certain stage in her development.

Around the same time, we had a breakout of internal parasites in our guppies which made their stomachs cave in, and we ended up treating them all for this. The aggressive guppy was one of the ones who died from the parasite. I have to wonder whether that might have had a connection.

 

[ninjacheesefish]

I'm assuming that the two guppies that you have are male. If this is the case, they're just being normal guppies. As you don't have any females in there, they're just being male. Guppies will fight to be the dominant guppy of the tank, so the behaviour your observing is 'normal'. Also having just a pair isn't a great idea, they aren't a shoaling fish but its best to keep a group of them together.
Either:

1) Buy more guppies

2) Take both guppies out the tank, rearrange the tank, and put both of them back in. This way it breaks up their terratories so it may discourage their behaviour.