I see three problems with algaecides:
- They are meant to kill algae which is basically a "primitive" plant form. So whatever is in the algaecide that will harm the algae *will* have an effect on all plant life. Sometimes this effect is minimal, sometimes not.
- More often than not they are "magical" snake oil products and beginners grab them and start pouring this stuff into their aquariums. When that doesn't work, they pour more in until the tank crashes.
- The very few products that actually work can become a trap: "Oh well, looks like that BGA is coming back... I'll just do another round of Erythromycin and that'll keep it away for the next x months"
. While this may work, I'd guess that even with careful and knowledgeable use of a proven product, long term continual use (even intermittently) can't be good for an aquarium.
I've always found that fighting algae is a two pronged approach: removing existing algae and fixing the underlying problem. Removing existing algae can be manual or with a product. There's nothing really wrong with using a tried and tested product (with known ingredients!) to help with the removal of the existing algae... as long as one is aware of the potential downsides and treats the product accordingly.
So far the only products that I have used that do the job on certain algae and when used correctly have a minimal negative impact on the aquarium are Erythromycin and Excel. I've never tried Hydrogen Peroxide so can't comment on that. None of these are snake oils.