So i have a fairly new tank, about a month old tank.. Started with 3 fishes a week ago as my lfs tested my water and said i was ready for fish.. added a green mandarin, royal gramma and mida blenny. 3 days later mandarin and royal were found dead near the area mida blenny hangs out in. Tested my water and everything came back good, no issues. So i can't find any reason other than mida bullying them because i did see their fins slightly nipped at.. Lfs told me midas are peaceful so idk what the reason could actually be for their deaths. Mida now swims freely all over the tank but recently I've noticed that when i turn tank lights on i find him sleeping on the sandbed and after a few seconds he continues on with his day and swims close to the surface.. Any chance this could have to do anything with not having enough oxygen? And if that might be, how can i help increase it..