What is going on with my upside-down catfish

[zeezMOG]

So about 2 months ago I bought one synodontis aterrimus catfish and 2 guppies, and the catfish was always good with the guppy and it was shy, as I started to learn more about them I found out that upside-down catfish need or like to be in groups so today I bought another one and it seemed that they were both soooo happy, I wanted to go to bed now but I saw my older one chasing the new one and the guppies, and I think its trying to rip there fins, and it makes weird duck noises every it tries. It already ripped one of the guppies tail. it never done that before what's happening? Its a 3.5 gallon which I know is EXTREMELY small but it was always nice with the guppies I thought it would love to have one of its kind? And they played a little when I first put the new one in. I am planning to upgrade to a 20+ aquarium. Plz help

 

[Cpt carp]

You have actually answered your own question. Bigger tank i suspect with the addition of new fish they haven't got room to establish their own territory thus have become aggressive