First Time Grower, Longer Term Toker

Plant 4 isn't doing too well. I think some bacteria started growing within the pebbles when they were exposed to the light. Had a funny smell. So I dumped the water and covered all the plants. Hope it recovers. Plant 2 seems to be doing the best. General Hydroponics nutrients for this week calibrated for 4 gallons of water: Flora Micro (2 tsp), FloraGro (2 tsp), Flora Bloom (1/2 tsp). Water pH is around 5.6-5.8. I have been checking the pH levels every other day and that seems to be right at the point where it needs to be adjusted to a lower pH to around 5.5. Nothing below that.