My experience with fruit flies... you have to remove whatever it is that brough them there. I have had fruit flies from
... rotting fruit (imagine that)
... little bits of food stuck in drains (chlorine powder/salt? packets helped)
... multiple damp kitchen rags left in a bucket for a long time
... a mop head that NEVER gets dry
... certain wet dog/cat food that has been out for about 24 hours.
You just gotta find what it is that they are going for and get rid of it. It's usually something pretty simple.
If you have something that is both holding moisture and rotting, preservative free, that is probably your source.
I've never heard of fruit flies going at grow media though. ?.
Did you drop a slice of pizza behind the reservoir? Maybe a dog/cat got in your garbage and dragged something somewhere?