Not sure if i have fungus nats or root aphids need some help please!

okay so on my plant while i was flushing i noticed a small black fly. the thing kind of looked like a fruit fly only it was longer. It came out of my drainage hole at the bottom of the pot. Also my plant is showing signs of mag def but i sprayed with epson salts and gave it cal-mag in the water so obviously it doesn't have the def. With this info can any of you distinguish between the two? also i have put sliced potato in the pot to see in 12-24 hours if anything will be eating it. I also made a red wine vinegar trap to catch any adults flying around.

any help is much appreciated!! if you need any more info i will do my best to give it to you.

yeah im pretty sure i have fungus nats so any good remedies to rid of them quickly?