I like mosquito bits for fungus gnats.
Mosquito Bits® When sprinkled into standing water where mosquitoes breed, biological Mosquito Bits® Kills Mosquitoes Before They’re Old Enough To Bite®! Easy to use – sprinkle or broadcast over standing water to kill mosquito larvae Quick acting, starts killing mosquito larvae within minutes...
summitchemical.com
Not sure if you can get them over there but they are basically natural and safe around waterlife like fish~~!
They work from a bacteria that kills the gnats.
Usually no more than 2 applications are needed.
Fungus gnats could [I say could not absolute] be caused from the media being too wet for too long, but if your plants are good no sense messing with that.
People also use yellow sticky traps for them as they only reproduce after you see them fly.
They live in your soil/coco/ media and that is where they do their real damage[ to the roots].
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