I doubt one drop of bleach in 15 gallons of water would do anything as it would dissipate in seconds. Having said that, (and it is a guess) I used bleach many times in my tank (without fish in it) to sterilize the water, then I would re use it,
I also used to treat my seawater (from the sea) with Regular Clorox, after I collected it to sterilize it.
I didn't invent that, it was Robert Straughn, "The Father of Salt Water Fish Keeping".
At the time, when the hobby started in the US in 1971 we were afraid of parasites and didn't know much about them and we lost many fish to them. Since then, I have the opposite thinking and don't care about parasites now.
He stated that the first squid eggs were hatched out in bleach treated water.
After treating the water with one Cup of Regular Clorox to 50 gallons of sea water, I let the water sit for a week, then added "Chlorine Remover" and used that water.
I did that for probably 5 years and never had any problems. I think it's in my old log book someplace.
If you have a tank filled with hair algae, aiptasia, ich or Godzilla larvae, Clorox Treatment would definately work although I realize no one here, besides me would do that.
