Guppie Fry Are Itching Anyone Know Why

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Hello evryone over the last week i have been waking up to dead fry in the breeding box in the tank and
now they are itching dose any one know why any help would be appreciated THANKS!!!

 

[WILDER]

Does the breeding box run tank water through it.
Check stats in ammonia,nitrite,nitrate,and ph.
Any meds or salt in the tank.
Check to see if you can see any salt like grain spots on the fry.
How many fry are in the box.