I received a new 5 gallon tank and have 2 fish in it, both males, i believe they are both fancy tail guppies, 1 i have had for a few months and is doing fine, the other i got a week ago and a couple days ago he stopped eating. i feed them tropical flakes and he acts like he wants to eat, but he either just spits it back out or doesn't go after it altogether,what do i do???? please help!!