Whoever told you it is harmless is nuts. I have experienced this and just raising and lowering the overflow box (assuming your tank is not drilled, which it seems it is not) will not solve the problem.
One or more of several things is conspiring to reduce your flow to your sump. What sort of overflow do you have? I have a U-tube and here is what happened to me:
I was getting air bubbles in there. If you have something physically blocking it, like algae or debris, that is easy, clear it out and boom, you are good to go.
This was not my problem. I kept re-starting the siphon. I ran an airline tube to the topmost point inside the U-tube, and connected the other end to a venturi. Worked for a copule days, then problem reappeared.
So I got an Aqualifter for $10, and that really helps suck the air and water through the tube.
But the real cause was not enough flow through the overflow. Why all of a sudden? Something was clogging my return. My spray bar nozzles were clogged, but the tube from my pump to the display was also clogged with gook. I cleaned it out, and now all is well.
In the box the water level should be at least an inch or two below the level of the tank waterline. That will give you surface skimming, and remove the gunk.
Identify and fix your problem or you will get poor oxygenation, algae growth, disease and death, if not right away over time. Identify and fix your problem or it will get worse and if it is equipment you will get pump burn-out, flood, or worse.
