HELP HELP MEEEEEEEEEEEEEEEEEE !!!!!!!!!!!!!!!!!!!

After my system cycled back in Dec 2012 my PH which was running at 8.2 - 8.4 it took 30 days before it hit 7.4
I then added quail egg shells which kept my PH between 7.4 - 7.8 and my plants were doing great.

In March the PH when to 8.2 - 8.4 all plants a doing less than OK, I removed the egg shells but can not get the PH down.

PH = 8.4
Ammonia = 0.25 ppm
Nitrite = 0 ppm
Nitrate = 160 ppm
Temp = 80 deg.

Can what I planted be affecting my PH levels? Maybe the onions or potatoes?