The hardest fish for me (and I could never understand why) was guppies. Everyone says they are the easiest fish to breed, and all you have to do is have water and guppies in the same house and the guppies will breed. I could just never get them to breed for me. When I was 9 a friend of mine had a tankful of guppies, and she would give me several guppies (on several occasions) but they would eventually fade away. Luckily I knew where to get more.
All this while I was breeding zebra danios, dwarf gouramies, and a host of other fish. I did eventually get them to breed enough for my award points. Never did figure out why they won’t breed for me.