If there were an algaecide available which would kill off algae without harming the plants or fish, it would be one of the most sold substances in the aquarium trade. You can be pretty sure such a material is not available. Excel comes close to being an effective algaecide, but at the concentrations where it actually kills algae, rather than just inhibiting it, it also kills some plants, and is in the range where the constantly emitted fumes from the dosed tank would not be healthy for humans to breathe every day. When we use Excel to kill algae, it is by directly applying it to the algae, not by routinely dosing the tank.