Alright.. well I just typed out a huge response, but then clicked back for some reason.. so.. more direct.
I thought over watering, but unlikely on a 3-4 day water cycle. Probably because of the hard soil, as aforementioned, lack of oxygen.
A few tips or suggestions.
1. Carbohydrate Feed (0-0-0), any hydroponics store will have this for ya. Organic versions are mostly kelp/molasses based, not sure of the inorganic versions, if there are such a thing. Basically an energy drink for plants, as you can see the n-p-k is a fat zero all around. Use at 1/4 strength at this stage is what i'd suggest. Hydro shop should have some free or very cheap measuring cups.. just ask if they have any free cheapies if you need a measuring container hehe.
2. Check this months Cannabis Culture or High Times, they actually talk about carb feeds during flush.. but they go more in depth.. however one of the magazines ALSO has an ask Ed section, and specifically curling leaves.. if its curling for no clear reason it might be genetically coded to do so (as a mutation or just strain characteristic).. doesn't mean its unhealthy.. however i'd suggest to exhaust other options first, to rule anything out.
3. Lastly, this stuff, I swear by it.. i've been using it for months, and its just a sample bottle, its called ALG-A-MIC. I've used it at all stages, except once i've started my flush cycle. Whats in it? I have no idea, when the plants are young I use about 1-2mL per 1L of water, veg to flower, about 2.5-5mL, and flowering up to flush about the same. HOWEVER.. its usually a splash at each stage, once when young, once when in veg, and once in early to mid flower. If I see nute deficiency its used first which almost always clears things up, if not then I move to fish emulsion options.
Again this is available at many hydro shops, and should be available online too.. and it is..
GroWell > Bio-Bizz Alg-A-Mic
I've stumbled into the BioBizz products slowly, and haven't looked back. I run everything organic when possible, or try to, except of course ph calibration fluid, which is debatable lol..
Anyways, no cash, no worries, if the plant looks like its hurting, a flush, a bit of fresh soil, and gentle soil loosening goes along way.. it might not be the best lookin plant in the world, but trust me man, it'll be the best herb that you've ever smoked
. Remember its good to treat them delicately but they are hardy!
Share the peace.
Bones.