Why is my guppies floating about in the same corner?

I have been observing my guppies for almost 6h straight! I found that the males are realy active.. Or atleast the shy one.. Realy wories me.. The rest (3) just float about in the corner.. Have a realy expencive filter.. More then enought 02 in the water.. Have no clue what the pH is.. I am abit woried about that but didnt get time to check that yet.. 160L tank.. I hav basicly only added Prime(seachem) .. Im new to this hobby.. They arent gasping for air though.. Am i puting them through torture by nt being sure about the pH??? I probably know the answer but they seemed fine a day ago.. ? HELP?