Is this fuel cleaner good as they claim it to be? Will it do harm to my car?

My car is an 03 Honda Element 4 spd auto with 140,000. Miles. Before I get my expensive mechanical work done I'm doing a trail and error process of why my acceleration might feel sluggish. Recently I have changed the air filter and when I took it out, it had different spots of black sutte air the filter. After I've changed that, the engine definitely breaths better and is more responsive to the throttle inputs I give it off the line and at speeds close to top speed.
Spark plugs were replaced at 95,000 miles. What other maintenance factors should I take into consideration?

Anonymous, answer this.

I can't see what fuel cleaner you hold in your hand, but none that are sold in stores will harm your engine.

The results will be positive, but negligible. You probably wouldn't feel the difference, but a dynamometer may.

Sluggish acceleration could be due to a bad transmission ($3-4K), bad fuel filter or a waning fuel pump (unlikely as Honda is known to put quality electronics in their vehicles, but still very possible.)

When did you last change the fuel filter?

The engine cleaner is not going to compare to a cleaner used by mechanics, but if you are comfortable doing a little engine work, you could clean the throttle body yourself.

Look up videos on YouTube.

Try seafoam and a new fuel filter.

That will not help much from experience if you drive a car in town only and if you put in some cheap fuel. My honda accord was sluggish, it would need 3rd gear up some small grade when I was filling it on some pioneer fuel station .It was cheapest station and it was on my way to work so why wouldn't I. But once I filled the tank with shell fuel after few hundred km that same grade would easily do in 4th gear. Now I prefer to drive 2 km to shell station to fill up and pay few cents more per liter. So before anything else that is what I suggest you do… Fill up full tank with shell and take a car on highway occasionally if you weren't doing it before.

Very Honda needs the valves adjusted every 60,000 miles because they get tight and you lose power and end up with a rough idle.