yellowing of oldest leaves?

Hi all,
Having an odd problem. Oldest leaves are yellowing. I already plucked off the two oldest fan leaves which were totally yellow, now it seems to be "spreading" outward. Help?

Setup description:

DWC, 1 plant.
Res: 73F temperature, 5 gal, 2 airstones
PPM: 900
pH: 5.5
Nutes: General Hydro, res just changed + 2tsp/gal hydroguard
Room temp: 79F
Light: Mars Hydro "300W" approx 18in from plant

You could give them a little epsom salts, tsp/gal, as judging by the other leaves that are dark green with brown tips they have more than enough N. For that 5 gal pail that likely only has 4 gal of water in it a Tbsp dissolved in some water and used to top up should work fine if it is Mg deficiency and won't hurt if it isn't.

Some strains just seem to like getting rid of the old leaves too so I wouldn't worry too much.

Good luck!