Is the humidity too low @ 50%?
No way, that is perfect!! Beautiful!!! No need for a humidifier with that kind of humidity. Only will increase the temperature in your grow cabinet.
I currently have a 250 cfu fan pulling air OUT from the tent through the cool tube, past the light. Should I maybe reverse that and pull outside air into the tent?
No leave that in there, matter of fact, you might want to add another fan to pull more out, typically this fan would have a carbon filter attached to it to remove any order. I have a 6" fan pulling through a carbon filter now and it works great. You need another inline fan that is pulling air from outside the cabinet to the inside of the cabinet in addition to the one for your light. Plants constantly need a resupply of fresh air (the need for CO2). Now the key there is to make sure that you filter this air. Pick up a cheap Hepa filter at Home Depot. I duct taped the filter over my fresh air supply and it works great, filters 99% of dust and other stuff.
I'm going to dump ALL the water out, clean everything with vinegar and h2o2, and start again. When I refill it, should I pH test and balance the water in the res. or outside in a separate container?
Perfect, get that junky root stuff out of there and clean up the reservoir like you said. As long as you are going to use the same water source, don't worry about PH right now. Take that meter you have and put it under your mattress for another week or so, that way you wont be tempted to use it. How are you supplying reservoir water to your net cups? I am assuming you are using feeder tubes?
Also, before I flush and refill, should I work on getting the temp in that tent down to a reasonable level like 70-75* first?
Absoultely!! Don't do a thing i.e...plant seeds, add a clone, nothing until that temp in that cabinet can get no higher than 79 degrees with everything running, including all lights, fans, pumps, anything....test your tent temp first and let it run like there is a plant in there for 24 hours so you can get an idea of your temps. Put a temp gauge at the top of the tent on the rails, check the temp about every 3 hours to insure that you are staying consistent. This is when the lights are on as well as off...when it is dark you need to check the temp also, you dont want it to vary more than 10 degrees...if the temp is 78 with lights on, then no cooler than 68 with the lights off. Think about the temps outside and what is ideal to a plant....high 70's in the day and hig 60's at night is optimal. That is why the mountains of Cali and here in Colorado are perfect for growing...great temps and great soil...but in our case, great water!!!
