Leaves curling upward

I have a plant that has the leaves sort of cup upwards on and off. The other two plants have done this as well but only once and never again.

Growing Indoors
Watering on average every 2-3 days
Generic Potting Soil /w additional peat moss, perlite, vermiculte and a conservative amount of composted cow manure
3g. Containers
Veg, approximately week 5
First 4 weeks were with only one 26w 6500k CFL per plant
6 Days ago I got a 600w MH running at 75%(450w)
depending on temperature I have canopy from 9" when at 70 degrees up to 22" when mid 80's
Light Cycle: 18/6
Fox Farm Trio for nutrients
I flush with pH'd water every other week to prevent build-up/toxicity
RH: is usually from 40%-70%
Nutrient Solution ph: 5.8-6.8 90% of the time it is 6.1-6.5
Daytime temps: 75-85
Nighttime temps: mid to upper 60s
Genetics: Bag seed from an amazing indica and two mostly sativa hybrids

I have heard that the leaves cupping up like that slightly is a good sign of vigorous growth
but, I am worried that it may be somthing else. If I need to provide any more information just ask.

Your plants are perfectly healthy, but the way you grow in soil is looks more like a hydroponic way, I assume this is the reason why they are so small for 5 weeks.
Ph must be in the range of 6.4-6.8, flushing soil is bad idea, as nutrient availability is very dependant on microbiological processes and by flushing you disturb these processes. Watering until you have a little run-off is enough to prevent salt build-up, and again because nutrients are mainly organic, there is very little chance of salt build up, no need to worry about it. For toxicity, just buy PPM meter and every time you water, take some readings of your nutrient solution and from run-off and record, from this readings you can decide whether nutrients concentration must be increased or reduced. Also depends on your soil, if it has slow realising nutrients, I wouldn't feed at all, just give plain water unless you see signs of deficiency.

Forgot to mention, don't water too often, let soil to dry out 2 inches deep before watering.

Yeah I had the pH on the acidic side because when I started them out it was just perlite/verm/peat moss. and I read that is considered soil-less which is the same pH range as hydro, then the 2nd tp I added a small amount of the potting soil to the mix and took the pH up a a bit, now with the current containers there is much more potting soil so i took it up to wher it is now. I will try switching to that pH rane however and see what happens.

Edit: they are also small becuase the first week was a 13w cfl then until 6 days ago a 26w cfl for each, since the MH has been introduced, they enlarged the fan leaves dramatically, increased number of fingers on the leaves and side shoot growth accelerated. I did super crop them last night however.

It's not a good idea to mix soil and hydro medium and use soil designed nutrients the hydro way. You could add about 20% perlite to soil for better drainage! Plants in hydro like moisture, in soil they like to stay dry, same for difference in pH. You might have quite a bit of problems with this grow.