First of all, I was giving a protocol for Fluconazole, not Fenbendazole. Very important not to confuse the 2. I’ve never used the later since it kills a lot of different things in reef tanks in addition to the target pests (usually clove polyps). Whereas fluconazole kills Bryopsis (and GHA at higher doses), but seems to leave other inhabitants alone as far as I can see.
Regarding the environmental implications of using antibiotics and antifungals in our tanks, there are more opinions than data on the subject. The bottom line is we don’t have the rigorous data or even scientific understanding we would need to know for sure how much of a concern it should be.
As far as the dose goes, it isn’t dissimilar to the dose that humans would take and then pee out, and is far less than the doses given to aquaculture or factory farm animals prophylacticly (for those that are given that way). Some humans are on chronic treatment or prophylaxis with these meds and are discharging them into the environment for years. As far as how much of the active molecule survives being in a tank biome vs human vs cow and into the environment, no one really knows (or at least we don’t have a comparative understanding of tank vs other). As far as how much it survives the waste water treatment protocols, bleaching, exposure to sun/UV, etc, no one really knows. And these important pieces of information would be different for every antibiotic/antifungal- it isn’t a reasonable estimation that if you know for one you know for all. We do know that none of these complex molecules last for excessively long times, they do degrade eventually.
When you do water changes, whether you have a treatment running or not, you should be discharging the waste water into the sewer system not a street drain. The sewer system will subject the water to various stages designed to break down complex molecules, kill residual microbes, and otherwise make it safe for discharge into the environment. Street drains generally don’t.
Fluconazole supposedly is bound by activated carbon, so in theory if you run carbon in a reactor after the 14 days or so, you’d be binding it up and not discharging into waste water with water changes. I don’t actually know if that’s true, and I tried to find data on it. Other antibiotics are thought not to bind to carbon. Most we just don’t know.
Some people argue that treating our tanks with antibiotics/antifungals that humans use could increase antibiotic resistance in human pathogens. That’s only a concern if there are human-specific pathogens in our tanks, which as far as I know isn’t the case since we don’t keep humans, other mammals, or birds in our tanks. Is it theoretically possible? Yes. Is it plausible? I don’t think so.
The bottom line is we don’t have the data we need to know for sure how much of a concern this should be. On the other hand, we do have a lot of very concerning data about antibiotic/antifungal overuse in factory aquaculture and factory farming. So if you are worried about this topic, you’d do much better to stop eating meat (including fish) every day than to stop treating your tank once a year.
