it sounds to me like they are attracted to that bad egg. i know the flies are attracted to moisture, but if they are there in force and laying eggs, there has to be something there for the maggots to eat ie dead eggs. i have never had flies attack my healthy eggs like that. the eggs i have currently incubating are in an airtight container. i just open it once a week to refresh the air and add water every two weeks( it never needs much water)
if they dont go away with the removal of the dead eggs you will probably have to carefully change that medium try a few hanging flystrips around the room too.
hope this helps, maybe someone who has had more of a problem with this will chime in and have more insight into this
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1.1 Gargoyle geckos
0.0.1 crested gecko
1.1 leucistic leopard gecko
0.0.1 asian golden tree frog
1.0 oriental firebelly toad
0.1 european firebelly toad
1.0 albino betta
1.0 orange tabby
0.0.1 asian painted frog
1.0 broad headed skink
0.0.1 fowlers toad
1.1 red eyed tree frog
1.1 red eared sliders
476.769.47 feeder crickets 