You telling me I'm not skimming because the bottom of the slit is an inch below the water level?
You are certainly not skimming to the maximum potential of the weir (or whatever you want to call that grill in this instance).
However, that is in no way saying that you aren't skimming enough.
Thinbear's pic is a great example of what I mean. if you consider the 'hollow' spots on each. One is mostly skim, maybe 80% of the total volume is 'skim'.
The other is mostly water and only maybe 5% skim.
Perhaps a tank only needs 2% skim. In that case your grill is beyond appropriate however it is not performing as good as it could. The best skimmer would be a weir as long as possible with a waterfall as thin as possible. probably not feasible with animals in the tank, but certainly the most effective at skimming. the grill has two negative effects on the skimming abilty of the weir. By adding the vertical bars you use up half of the length of the weir and then you cause the water to back up and make the crest of the waterfall too thick, which is the opposite of what you want to accomplish.
However fish have issues with a nice smooth weir. The water is always clearer on the other side it seems that they just can't resist the temptation.
The skimmer I described further up is almost exactly what thinbear drew and works brilliantly. I haven't tried it in a fish tank (and do harbour concerns about fish jumping in) but I have used it to skim in other applications and using a 500ml beaker I had great success.
And just for a quick credential, my weir skimmer knowledge is from training I have received in watewater management.