I use blackpowder every so often but I make sure to target feed my corals with a syringe you can get for medicines. They have no needle. Just a blunt plastic tip. I premix the powder with some tank water then I feed away. But I also do this proceedure with Reef roids.
I would recomend that you don't just sprincle it on the top of the water. They claim it will just gently sink in the tank over afew hours and better feed your corals. This was bad, firstly if you have an overflow, Unless you turn off your sump for a long period it will all go to your sump and float there. If you don't have an overflow you will just have this stuff floating on your surface for afew days. I used it on my Nano tank and had to throw my spare aqua clear on it just to help push the powder down into the tank.
But I did notice a difference with my zoos mainly.
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