You will need to fill gelatin capsules full of it. Then you may bury under the sand preferably straight down to the glass. It would be hard to bury pebble by pebble under your plants. As for algae read up in the algae section, fertilization excess is probably one of the last things to worry about after co2 and lighting. As far as ammonia spikes killing fish, if your plants and filter are good I wouldn't worry. Others will probably be more helpful and more knowledgeable in regards to this topic.