Isn’t that supposed to be used for tanks that have a liquid Co2 system?
No, it's advertised as 'liquid carbon', supposedly for tanks that don't have CO2 systems, since those wouldn't need another source. Or in addition to CO2. From their website;
"Use as an alternative to CO2 injection or in addition to it"
But in practice, it's glutaraldehyde. This is a very toxic substance, and it's more of an algaecide than 'liquid CO2', which isn't a thing. It works in the sense that it kills off algae, so plants can sometimes grow better without that competition, but it also kills plants like vallis, and it's very dangerous when overdosed. Since it's also dangerous to humans, and so dangerous to organic life even in relatively small amounts, most of us avoid it, because who knows what damage it's doing to livestock even in the recommended doses. Given that fish literally breathe in the water, they're not just swimming in it, and the benefits are negligable, most of us find it not worth the risk, especially since it doesn't really work as advertised.
I used it twice, as an algaecide, and it worked to kill algae and I didn't see any bad effects on fish. But I won't ever use it again because who knows what damage it might have done/do to fish, and it doesn't solve the underlying cause of the algae problem either, so you still need to fix that, or continue using a toxic substance in your tanks...
