Unless you're water is as hard as a rock, that may not be the problem. It's very unusual to have absolutely none of the eggs be fertile (due to water hardness). Without insulting your fish-keeping abilities, have you ever actually watched them spawn? As was already pointed out, females will often spontaneously spawn, even without a male in the tank. The females ovipositor is very short, wide and blunt, while the males sex organ is a bit longer, very narrow, and pointed. If you are absolutely sure you have a male and female, and both are participating in the spawning process, and none of the eggs are fertile, I would check your water hardness first. If hardness is not the issue, then it's possible that your male is infertile.