Yee Ger! That looks like what you've got then. The leftovers of the gnat problem... Gnats aren't too bad mate, if you're gonna have bugs I'd choose gnats any day, they don't really bother the leaves too much so if there's a few buzzing about the place it's not good, but it's not DOOOOM like thrips or spider mite.
It's too late for this tip now but the reason I use vinegar for pH down is that it deters the gnats from laying eggs in the soil. (This is a tip I got a few years ago from an old timer called Duke, he knows stuff!) Lemon juice or pH down won't help this.
Where about sin the world are you mate? I'm UK. There's a product here called Gnat Off that can be used at any time in the grow without affecting the plants. I've used it, it works. What you've got to bear in mind tho is there might be unhatched eggs in the soil, so it's best to do what it says on the bottle, then do it again a couple of weeks later.
Tanlin is good too.. [emoji111]️