Ok so my plants are about a week old and two of them have small brown spots on the tip of the leaves...what could this be? Over watering? Overheating? The other question is when do I start using fertalizer? And i dont understand the PPM system....

If you are using tap water its probably the chlorine in the water. Thats what usually happens. Its ok. Its not going to die. It could be from over heating. What kind of light are you using? I feed my plants basically one week after rooting. It needs to be half strength or less. If your not sure then use 1/4 strength at first. Its better to be safe than sorry. What kind of fertilizer are you going to use? And what do you not uderstand about parts per million? Its the amount of salts that are in the solution.

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Ah see I thought it might be the water...but I let it sit for 1 day before watering...oh well. The lighting cant be it..I'm using 2 145 floresents but gonna start them on 2 450 HPS lights soon. Im thinking mircal grow fertalizer. Still need to go to the store for fertalizer. Thanks man