garlic powder is not the "same" as fresh garlic. and chopping garlic is not the right way to use garlic....thats why they have garlic press. you have to crush the cells in the garlic to release the oil that contains the strong flavor of the garlic. as for the oil in the fish tank...i dunno. the oil might be the only thing that actually sticks to the food after u drop it in the tank....powder might just wash off and foul the tank water.