Alaz:  Dissolving gold with cyanide requires oxygen;  without oxygen you will not dissolve any gold.  This is why you aerate your CIL dissolution tanks with air or if the O2 level is low you can add some O2 along with the air.  Air only contains about 19% O2 depending upon your elevation.  The most likely reason your no 8 CIL tank has lower oxygen than your no 2 tank is that the oxygen is being consumed with the presence of some sulfide minerals and oxidizing the sulfide to sulfate.  Some sulfide minerals are more apt to consume oxygen than others and fine grinding will expose more surface area and increase oxygen consumption.  Pyrrhotite will oxydize more readily than some of the other sulfides such as pyrite, sphalerite, or galena if you have them present.  If your dissolution rate is a bit too low in your CIL tanks, increase your oxygen level by blowing in more air.  I would check your complete tank profile (no 2,3 4, 5,6, 7, 8) for oxygen levels and add air or some oxygen to the tank where you are starting to have lower levels.