There could be a few possibilities here, one and foremost, it sounds to me like a stress issue. Guppys are "busy" fish, and bettas prefer to be alone.
You say your water params are "fine"... could you please post the readings for ammonia, nitrite, nitrate, and pH?
What kind of food are you offering to the betta? What is the water temp?
If you watch the fish for a while, are your guppys chasing around a lot? Do they chase the betta and nip at his fins?
The more info you can offer the faster we can help.

 

I don't have the test strips with me so I will give you exacts on the water tomorrow. The guppies keep to themselves. It's almost like they all ignore eachother. I give the guppies Omega One and the betta gets Betta Bites. He might take a bite of thier food but normally spits it out and waits for his own.

 

I wouldn't bother with the strip tests. They are extremely inaccurate. If you don't yet have liquid test kits, try taking a water sample to your LFS and see if they can test it for you. Ask them to write down exact numbers for ammonia, nitrite, nitrate, and pH. Also ask them to write down what kits they are using.
One more option for you... move the betta to a fishbowl by himself. Keep him in a warm place so his temp stays 70 or above, keep his water clean (50% water change every 2 - 3 days), and feed him every other day. Watch his behavior. If it changes for the better, then you know stress is the issue. If it doesn't, we can begin looking at other possible issues and how to treat them. If medication is needed, this will be safer for the guppys.

 

Thanks for telling me about the strips. I didn't knwo that! Here's what the strip told me since I already did it this morning: 60ppm GH, 80Pppm KH, 6.7 ph, 0 nitrate and nitrite. The water temp is at about 76-78. I'll pick up a real test on my way home tonight.

 

Don't forget the ammonia test. If you work with the master test kit you should get ammonia, nitrite, nitrate, and pH. These will be the most important to monitor long term.