little yellow spots on leafs

My friend has a hydro set up and the level of food hes putting in is correct an the ph level is always checked and its neutral , and he says he cant understand how there is little yellow spots on leaves even though they are growing fine . He doesnt think its spider mites as there aint any little black spots or webs anywhere so what else could it be?

You'll see webs when the mites are fully entrenched. Look under the leaves along the center veins. Use a magnifying glass. If you have mites, you'll see them then ... or at least you'll see their eggs.

Vi

I thought when you ran hydro PH should be slightly acidic below 7 neautral more like 6-6.5 or even lower maybe there just being picky (the plants)