betta aggression

Question
My beta seems a but aggressive, is there anything I can do to stop this aggression? Well, I'll start off by saying
thank you for reading this and answering my question. Well, I had a beta and a bunch of snails before I added a small blue guppy. Before the guppy, the beta seemed vary happy. The beta never bothered the snails. I purchased a guppy that doesn't seem to bother the beta. The betta will chase the guppy around the tank. Before I put the guppy in the tank, I had him in the bag he came in laying in the tank(to get used to the betta/get used to the enviorment of the tank). When the betta chases the guppy, is it aggression that I should be conserned about?

Information to help you answer my question:
My tank is two and a half gallons, the temperature is usually in the high 70's to low 80's, and the tank does have a filter. The tank water is clear/not cloudy. The tank has a bunch of small snails. I feed the fish a little in the morning, a little in the afternoon, and a little at night. The tank is just under two months old, and has not had the water changed yet(it hasn't shown signs of needing the water changed).

Answer
Hi There Katie,

Thanks for taking the time to ask a question!  I'm sure your fish appreciate it.

Now, the problem I think you're having is definately agression.  The betta, used to being alone, is feeling threatened by the new addition to "his" tank.  The snails aren't a problem because they're small and don't move much, but because of the guppy's big flashy fins and quick size, the betta sees him as a threat, if not a smaller male betta.  That's why you're getting the problem.  Unfortunately, the only solution for this is to remove one or the other fish, or purchase a larger tank, as with more space, the betta will no longer feel his territory is in danger.  If you don't separate them or give them more space, chances are the betta will harass the poor guppy to the point of illness or death.

A couple of things worry me, however.  Your temperature is a good level, but weither or not the water looks like it needs cleaning, it should be done at least once a week.  Fish poo and pee creates ammonia in the water, and that can severely harm your fish, and even lead to worse conditions.  As well, that you just added the new fish to your tank without quarantining it isn't a good idea.  The new fish can transmit diseases to your tank and endanger the lives of your other fish.  I would keep an eye on Mr. Betta and make sure he doesn't get sick.  If he does, be sure to come back and ask one of the experts for any assistance you might need.

Hopefully this answers your questions, if not, be sure to follow up with anything else I can help you with.