Excellent water conditions is one of the keys, yes. But so is stable parameters. With my next batch, I'm going to soften my water by doing 50% RO and 50% tap. I'm also using Wisteria in the tank to take care of any nitrate increases as well. I do not do water changes as when I did, I would lose over half of the fry, even with less than 10%.
Also, another change is I'm going to a 5G for a fry tank. Still have to get it, bonus time is Wednesday, so I'll splurge and get a 5G setup with small filter and heater. Will be bare bottom, and will try the blue stuff this time for the anti-fungus. I'm also leaving lights off, so the only light they will get is ambient lighting. Will see if this makes any difference.