It will be virtually impossible to avoid algae with inconsistent CO2. Algae is not dangerous to your fish. Algae can, however, take over your plants by attaching to them. The only cure for that in most cases is to prune off the affected leaves.

Yes, CO2 lowers your pH -- with such hard water, however, your pH will be more stable and not lower as much as in a location with soft water.

Water can be too hard for some fish and plants. You might have to choose your critters and plants accordingly.