It’s hard to have a 3W handheld laser built for $150. I built a laser. I used a laser diode from a DLP projector, flashlight, and some 18650 batteries. It also required some custom circuits I won’t disclose for safety measures. It’s not a toy. I’m not sure of the laser power output. I’m sure it’s north of 2W. A laser that pops a balloon or burns a paper towel is around 150mW. Mine will set the grass on fire, light fireworks, ignite gunpowder (all of these from 10 feet away). I have laser goggles and it’s still absurdly bright. It runs for about 40 seconds before the batteries are dead.
Think about the physics involved to “burn” pests off a rock in an aquarium. How thick is the glass? How much water is in the tank? How far is the rock from the glass? Saltwater or fresh water? All these things effect the beam at the point of impact. There’s more physics going on here than there is for a 2500 yard rifle shot! Light dispersion through water and the thermodynamics of the energy transfer through said water are some hairy math which I don’t care to ever do again. Also, the minerals in said water will have an effect on the beam, so the wavelength of the light becomes a factor.
Additionally, to measure your laser’s output power, you need a very costly laser output power meter, or you’ll have to design a precision experiment.
All this said, my guess, to burn said pests in say a 100 gallon fresh water tank, where the rock is approx 8” from the 1/4” glass, you would need to focus the beam to less than 1mm, and have a 800nm laser power of >7W. Since that’s an IR beam, you’d want an aiming laser as well (5mW red). A 7W laser needs a huge cooling system as well as a massive current source. Remember, lease diodes are sluts for current. They’ll draw as much as they can until they burn out. The must have some kind of current limiter.
Get a refund if you can and clean your tank the old fashioned away. I know it sucks and you’re afraid of disturbing an ecosystem, but unless I see someone do it in person, I have to believe it can’t be done.