I have a single female angel and she used to lay eggs on my filter intake every other week. First, I would notice her guarding the area and meticulously cleaning it. Then she would lay eggs for about 6-10 hours. Then she would guard the eggs as long as she could, but during the night my pleco would get them. Obviously they are infertile anyway since I don't have a male, and if any remain they will turn white and fall off, or be eaten by the angel or other fish within the next day or so.