Is this problems from Fungus Gnats ?

Fungus gnats don't cause big mutations in plants until they get into an infestation stage. like 500-1000 of them. The best treatment for gnats is beneficial nematodes, the brand I buy is called pod poppers. It eats their larva from the inside out. within one-two weeks their population drops by 98%, at least for me. That in combination with yellow sticky traps is great. Gnats also only become a big problem if to much watering is going on. maybe try watering less or from the bottom up for a week or so. As for pH issues. You can test your mediums PH buy getting a sample of it. like 1" cubed. Toss that into 1/2c of distilled water. Shake it up well for 5 min and let it sit. The next day give it a 2nd shake and let it settle, 1-2hr, put your PH pen inside the liquid not the settled wet medium and try not to disturb it to much. That should give you a basic PH reading of your soil. You can then adjust from their. After u figure out your PH, you can make a better estimate if your plants are locked out of some nutrients or if they are being overfeed. Top yellowing from centre out on new growth could be a sign of Iron and or Sulfur deficiency, or maybe light stress but I doubt it, PH might be locking out the Iron or Sulfur. as for tip burn, that's normally to much nutrient related. best of luck

Others plants just look norally but still able to see a few of gnats