The most likely reason the leaves are yellowing is because the plants have used up almost all of the availible nutrients in the soil and are begining to show Nitrogen deficincies. Nitrogen is the most important nutrients in good cannabis growth however not the only one. If you want a inexpensive nutrient solution I would tell you to use Peter's Proffesional which can be found in any home improvment store. It must have a NPK ratio of 10-10-10 but no higher than 20-20-20 which can be found on the front or back of the bottle and always remember cannabis can burn easily by using too much so always mix the solution at quarter or half of what the bottle says. In addition try to use only distilled or reverse osmosis water as most tap water is hard and contains unsafe amount of impurities that are bad for plant growth. To your last question do not clip off the dying leaves but let them fall off on their own. Clipping it could cause your plants to go into shock which inhibits growth (Stunting) and will raise the likelyhood of it turning into a male or hemorphidite plant neither of which you want.

Good luck to you and I hope my input helps.