I found an old bottle of Aquarium Product's discontinued dechlorinator Genesis. It is the same chemical dilution of sodium thiophosphate as Prime, just twice the strength. Their recommendation is one drop per gallon, with no increased dose when added directly to the tank. My suspicion is that Seachem is erring on the side of caution. In heavily chlorinated taps or those that use chloramine one would think the risk of putting it directly in the tank would be significantly higher. Having more chlorine neutralizer in those situations would only be helpful. So never mind my mini-rant earlier. Do it however you are comfortable. Adding it only for replacement water works for me and I see no reason to change what's worked, but no harm in using more than needed. I'd imagine the dose it would take to hurt your livestock would be ridiculous.
Nothing good happens fast in an ecosystem.