Overtreating will do just that, killing everything - including fish. Even though most of the treatments we sell are safe for fish, if you kill off all the vegetation, you'll end up depleting the oxygen supply. And that is what will kill the fish. Of course, the best advice is to catch pond weed and algae problems early. Also, determine if your problem is algae or weeds.
Most products are effective only against one or the other.
That's why, Williams notes, pond problems often require the one-two punch of a combination of products for a total knockout. Use one of those products with Cutrine, an Applied Biochemists product that controls algae, and Rodeo for above-surface weeds. That should take care of most problems and inhibit re-infestation. Another treatment he suggests turf experts investigate is LakePak WSP, a biological lake clarifier and deodorizer from Becker-Underwood. The proprietary bacteria compete with food sources to reduce scum, sludge and foul odors.