looks like it could be a lot of things. from my experience i would make sure my PH is stable. for me the most important care giving tech. i use it taking critical care of my ph, if your ph is off then your whole grow will be off, sometimes starting over is what needs to be done. not only should you ph your water before you water, but also ph the runoff of the water. i cant get at it enough that proper ph is sooooo important to the plants.
if overwatering is your problem check out this link man
https://www.rollitup.org/marijuana-plant-problems/423760-overwatering-plants-anonymous.html#post5569246
for me, the best way to judge weather a plant needs watered or not is to let them dry out till they start to wilt a little, water one of them until you get some runoff (test the runoff) and then weigh the pots out in your hands. you will then get the feel for a plant that needs watered (in that size pot) and what a plant feels like when its just been watered. after that you can then figure out how much water you need to give them, and how long it takes for the plant to use up that water, remember as the plant grows it will use more and more water, but it is a very gradual thing.
for me the rule of thumb is 1qt of water per 1gallon of soil- depending on the size of the plants ill need to water once every week/week and a half. 1cup of water per 1 16oz solo cup depending on the size of the plants once every 3 days. (if you start in 16oz solo cups be prepared to transplant after 10-14 days. the smaller the container the more often you will need to water. the smaller the container the sooner you will need to transplant (in order to keep your plant at 100% giving their roots room to grow is so important!)
i loved my babies to death my first try, sometimes you just gotta take your losses and start anew. good luck bro, if you need any help let me know
if money is a problem, try these
http://www.amazon.com/PHion-Balance-Diagnostic-Strips-90-Count/dp/B000VRU4US/ref=sr_1_1?ie=UTF8&qid=1302274495&sr=8-1
very accurate for being as cheap as they are.