Ha! Don't worry, you're not the only one who thinks i'm crazy using this.
Draino is "100%" sodium hydroxide, sure it's probably contaminated with other stuff, but the quantities you are adding are so small it's insignificant.
Why do I think it's safer than Kalk? well kalk does not dissolve much, so there is the potential for fish to pick at the paste, this could cause chemical burns for the fish, due to the low pH. Sodium hydroxide on the other hand is extremely soluble, forming a strongly alkaline solution, (pH close to 14) because it's so soluble, any which is not target injected near or in the aiptasia soon dissolves in surrounding water, being neutralised into sodium chloride (salt) and water.
It's also more effective than kalk because the localised pH is much higher.
Layton