Little brown spots on lower leaves and yellow newer.

Hey Guys,

Started noticing new problems with my plant, little purple/brown spots on older lower leaves that are starting to curl up a little, unfortunately i couldn't get a good pic of that due to light but i did get some of the top which shows the upper yellowing.

Also i have not flushed yet when i replace the solution every week, i water it every 6 hours for 5mins. Do i flush by replacing this water with newer diluted stuff for awhile or do i just take a bucket to the pot and let it all drain out? (maybe its a toxic buildup?).

Any help would be great, im still pretty new at this.

PS: i also noticed the leaves that are the most sick are its original leaves it had from cloning with no buds or anything, do they just die off after awhile maybe?

E-indoor or outdoor---Indoor
E-soil, soilless, coco, aero, or hydroponic---Hydroponic
E-specific medium---Perlite
CSL-Soil type/brand---N/A
HCL-Hydro/aero/soilless system type---DWC
SCL-Anything you have added to the soil---N/A
SCLR-Soil or slab runoff pH---5.5
E-Water source---Small Tub (40l)
E-Source water pH---5.5
HRT-Source water EC (if hydro)--- 0.6
E-Age of plant---20 or so Days
E-Type of fertilizer--- Hydromino Accelerant
E-Rate of application (if hydro, this is your PPM number, preferably after each component is added)--- 1.9-2.2 EC usually
E-Lighting source and distance from plant---400 watt HPS, 30 cm away
E-Air temperature (both day and night if you are running a dark period)---20-24c light, 16-20c dark.
HD-Reservoir temperature--- Unsure, room temp prob, 15c ish?
E-Air % Relative humidity---50%
E-Lighting schedule---18/6
E-Type of ventilation your room has---fans
TR-Did you pre-soak your media in pH corrected solution?--- No, uncorrected tap water before i got a PH tester/PH down