Hello All...first, thanks for all the good advice

. I'm getting back into the hobby (with my 11 year old son) after a 15 year break and fish tank forums has been very helpful!
Can anyone tell us why our female guppies are getting sick, either just before giving birth or shortly afterward? They get a slightly swollen light colored band about mid body.
We've got three adult females left and about 30 babies now in a 10 gallon tank. We're using a UGF and a small Whisper power filter. A 20% water change is done once a week. Tank has been up and running for about 5 weeks. Ammonia is zero, Nitrites are zero, and Nitrates are 20ppm. We add a teaspoon of salt per 5 gallons of water and remove chlorine with Amquel. Water temp is kept at 78 degrees.
Everyone looks happy and healthy until its time to have babies??? We can't find any information on this condition

. Can someone help?
Thanks!