Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If the livestock has been moved, then go for a bigger water change, if you want. I have often seen the suggestion for 50% water changes, repeated, and wondered if there might be a reason.
Livestock in the tank is the most obvious thing that came to mind.
But a more complex idea came to me, also.
I wonder (seen no studies) if the stuff you are removing (fenbendazole in this case, but could be anything) might stick to the surfaces (surfaces are covered in biofilm, and are slightly sticky) as you drain the tank, so that going with a bigger water change is actually counter productive. More than 50% often exposes a lot more hardscape and plants.
If the chemical stayed in the water (was not given much of a chance to stick to the surfaces) then 2 water changes of 50% actually remove more of it than a 100% water change which exposes a lot more surface area for stuff to get stuck on.
2 x 50% actually results in a 75% water change.
In this case we are discussing a medication.
For medicine to be effective in an aquarium it needs to be sticky enough to stick to the target organism, even if the target is not so sticky.