We have a 10 gal cycled tank with 3 male guppies. It used to be 4. A few weeks ago I noticed one of them was racing around at meal time with the others but not actually eating. Occasionally it would nip a flake and spit it back out. It did this for about a week and a half before it died. Two days ago I notice the same behavior in another one. What might be going on here? I'd like to prevent another death if I can.