I have 4 guppies in a cycling tank. The tails are disappearing on 2 of them. The other 2 are fine. What is up?

I have a cycling tank with 2 gray guppies and 2 fancy guppies. The fancy guppies are the ones with the ragged tails. One of the gray ones is pregnant. She seems a little aggressive. She chases anything that gets even a little close. I have read about fin rot but wasn't sure because some of them are fine and didn't know if I should put any chemicals in a cycling tank. I have had the tank for 4 to 5 weeks. Right now the ammonia and nitrate levels are high but not extremely. I have been changing the water weekly (about 30-40%) and vaming the gravel. The fish seem to be doing well except for the tail problem. I have a 5 gal. tank with a 3-stage bio-wheel filtration system. (I know I need a bigger tank if I'm having fry). This is my first tank and I have been trying to read everything I can to make it a good life for these fish. Thank you in advance for all your help!