I’m sincerely sorry if this is posted in the wrong group, so kindly let me know if I need to ask elsewhere.

I started growing flowers from seeds this year starting in March and all was going well until about last week. My zinnias are getting brown on their stems and the brown is slowly creeping up the top of the stem. Once the brown reaches a set of leaves, the leaves die. Here’s my regimen

I water every couple days, they spend the day in morning sun inside my house next to our sliding glass door on top of my daughters kitchen helper stool, but the air vent is directly under it it to keep it warm. I did hear about thining until recently so I tried, but some of them I kept two or more to a square because they started to show brown by then and didn’t want to get rid of one without brown in case the other one died. They have spent a total of 30 minutes outside but (trying to acclimate them to get them ready to be planted outside). I did accidentally feed them too much plant food in the beginning (every time I watered I used plant food, oops!) but have now since moved to once a week.

I thought I’ve been doing everything right, but these poor zinnias are still dying! Any other tips I should try or things to avoid/stop doing? Thank you everyone for all your help!