Help! Nute Burn Or Something More Sinister?

I'm growing two plants under CFL's in a homebrew hydro tub with rockwool and hydroton, General Hydroponics Micro and Grow formulas (using the "vegetative stage" mixing ratios) through a drip system, and I've got an airstone inside the tub. The plants sprouted March 3rd; one is about four inches tall, the other is coming up on five inches.

Wednesday evening I got home and noticed some discoloration on a few leaves. By the next morning, the area of discoloration had grown considerably. The leaves themselves feel rough, somewhat brittle around the affected areas. Thinking it might be nute burn, I dumped my nutrient bath and spent a chunk of Friday and all of today flushing the plants with PH'd water and no nutrient. The affected areas of the leaves I initially noticed this problem on seem not to have spread, but the edges of a few leaves on each plant have gone white and/or tan.

Another source I've read suggests that the problem is a magnesium deficiency and I should treat the nutrient bath with some epsom salts, but I'm afraid of frying my plants with it. Any suggestions as to what this is and how I can make it go away, or at least stop?

If you're using a commercial nutrient mix, it's unlikely you'd have a Mg deficiency. I'm with your fert burn theory.

If you haven't got a nute meter, it's time to get one. The Truncheon is calibrated in ppm, EC & CF and has no moving parts nor digital displays. Waterproof and durable, but not the cheapest meter you can buy.

If you have solved the problem, all new growth will be normal, but dead parts of leaves are not going to heal.

Well, I've noticed some odd changes with my plants.

Plant #1, which was most affected by this malady, seems to be recovering but I'm not 100% sure yet. The leaves showing this problem origianally are dry and almost to the point of falling off. They look terrible. The leaves above them show very small signs of this same problem but the discolored areas do not yet appear to be growing. The upper leaves whose edges started turning white are doing fine, and the discolored areas are not growing here either. New growth has slowed considerably, but vertical growth of the main stem has accellerated for some reason. The root system, which is nice and white, has passed nicely through the second mesh pot and it already down into the water.

Plant #2 is having an inverse reaction to that of plant #1. Vertical growth has slowed to a crawl but new leaf sets are popping up in every internode and seem to be growing rapidly compared with the other plant. The two or so leaves affected on this plant are also looking a bit crispy but the problem doesn't seem to have spread anywhere else with this plant. It's root system had originally far surpassed that of plant #1 before it was transplanted to the hydro tub; these roots have yet to grow past the second mesh pot and into the nutrient solution.

I've noticed some insects, just some mosquitos or something like them. They seem to enjoy my whitefly traps far more than my actual plants. This pleases me to no small degree

I am fighting a temperature/humidity problem, however. This growbox is in my walk-in closet, which is on the second floor of my townhouse, somewhere in Northern California, where we are already experiencing outdoor temps in the high eighties to mid nineties. As you can imagine, it gets fairly warm in there, even with the AC turned on (the AC doesn't vent into the closet at all, so the closet door stays open all day). I've just installed an 80mm exhaust fan in the top of the unit, which vents through some dryer duct with a non-reflective interior. My temp is averaging 86f, which I'm sure is a bit too hot for these little plants. The humidity stays between 30% and 40%. Even with a small pail of water in the grow box I can't get the humitidy up any higher. I'm considering digging the humidifier out of my garage but I'm afraid that will only add to the temperature problem. I may build a makeshift vent system with an 8-inch muffin fan stuck in the top of the box.

HELP!

With aircon, you're going to lose your humidity as part of the bargain. Low humidity is much less damaging than excessive heat, tho.

Hang a breeze box fan from the top of the closet door frame, blowing warm air out of the closet. Close off the cool air vents in all other rooms, directing all cooled air to the room housing the closet grow op. That's a temporary solution.

If you have access to the attic through the ceiling of your closet, you may be in luck. If you can cut a hole for a typical bathroom fart fan (10" perhaps), you can vent your heat into the attic and draw in cooled air from the rest of the house. Your grow op scents will go into the attic too... which may not be so good if your neighbours share your attic airspace, There may be a firewall between each townhouse, though it may not be scentproof. Think about a UV ioniser or activated carbon filter for scent control.

And since humidity is relative, once you get the temperature down, the relative humidity will be just about right. VV

As luck would have it, there is attic access in the closet, and there are firewalls between apartments. Like you said though, I'm not sure if they are odor proof. I know for a fact that the people who live(d) on either side of my place grew indoors - one guy actually used an entire room for his grow in much the same way as that British "how to" video that's been all the rage on Youtube for a while now. We never smelled a thing, and I know he was only using Ona and incense and things like that to control the odor.

I've got some super-rigid cardboard in the garage; I'm going to mount a fan in a piece of it and stick it up there today.