a different material does not tell us the medium, thing is your roots probably haven't spread throughout the pot, so possible nute build up. if it drains fairly well, then i would put fresh water in it every other feed.or fresh water first alow to drain then another with nutes added.
wondering why you went straight to 15 gal and not step up in size as needed.
It allows more control of medium saturation.
when you potted it you felt the weight, so when you watered it and drained, what is weight diff.
and you can tell how much water is being used daily by lifting, i know 15 gal is a bit, one reason to work your way up in pot size.
But putting in just fresh water to flush is better than over feeding and having problems later.
keeper green