I got this guppy to be tankmates with my baby betta in a 10 gal tank three days ago but I today when I checked on my fish I found the guppy dead! Now first the betta didn't seem to like the guppy when I put the new bag in the betta's old tank so I put a divider up that blocked any fish from escaping it's side. This is a tank I used previously for reptiles but now it was empty and I used it for an aquarium. Anyways I got a filter, a self adjusting heater and I treated the water. 1/3 of the rocks I have used for fish in the past so they surely have helpful bacteria on it and same for two of the decorations. I didn't use soap for cleaning and I didn't put any fish deadly materials in the tank. The betta is still alive and well but the guppy only ate a nibble when I first got it so I attributed that behaviour to being in a new tank, also with the fact the guppy looked a little stressed due with classic new tank behaviors. There are no visible diseases, the fish didn't look emaciated, not visible injuries or low activity level. It's just like the guppy died with no know cause. Does anyone know what might have happened? I am worried for my betta whom I named Apollo, also Apollo wasn't constantly threatening the guppy but stayed in his own space doing his own thing.