Help guppy is fighting?

I have A 10 gallon tank when I first got the tank I had put for guppies and there are three of them had died but one was still alive my friend recommended me doing some different things to the tank since the one fish is alive to see that if it live for a couple more months after the different things and changes she had told me to do that I could bring more fishing at the take last Friday I went to Petco and got three more guppies tonight I m noticing that the one that was in there the longest is now lunging at the three new ones
I don t want to have to get rid of any but I don t want the other three to die just because of him acting this way any suggestions would be great

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