Nutrient salts are going to accumulate in any media that you run the nutrient solution through. It would be the same with a bucket of dirt.

Just watch your plants and see if they are showing the effects of too high of a nutrient concentration when they shouldn't be. You may have to flush out the rockwool with pure water every so often.

I performed a small test...

1 - 25 Lt. pail of nutrient solution at
PH = 6.0
EC = 2370
ppm = 1178

Add 1 ml phosphoric acid
PH = 5.8
EC = 2388
PPM = 1194

Add another 1 ml. of acid
PH = 5.6
EC = 2393
PPM = 1197

Electrical conductivity increases with PH level decreasing.

Maybe your tap water has a lot of calcium/lime in it and as the plant takes it up the PH increases.

Cliver,
wouldn't you reach a similar though inverse conclusion if you added calcium or potassium to raise the pH?
After all you're adding a compound to the water to lower the pH which will also, naturally, increase the EC as you're adding a dissolved solid to the solution.
buster,
I think your first assessment is probably correct. your plants are likely transpiring more water to cool off than they're consuming nutrient thus the run-off water has a higher level of dissolved solids.
Ideally, you'd want any run-off the have an EC of zero (meaning all the nutrient is consumed) but I think having the in and out solution match or the out slightly lower is a more realistic target.

PH is stable

drain to waste ph in res never changes since the water is always new and never recirc

so the ph isnt likely to have anything to do with the EC rising ... i set the ph at the start of a new res and im done never to ajust it again until the res runs out

as for over feeding.. i have tried feeding as high as EC 1.8 and havent seen any sign of toxicity or leaf curl except i have seen less fruits with taller spindly growth from the higher EC.. therefore im not feeling confident o leing the plants tell me cuz there not telling me there just producing less

i suspect the plants just stop obsorbing/transpiring as much fluid in order to reduce the take up of the higher than ideal salts = reducing mineral uptake =reducing flower set and yield as well as the high ec having high N levels for example on peppers high N = less fruit

I wouldn't think pH would matter, in absolute terms, to EC. That's because pH is a measure of H+ ions. In practice we are looking at H+ and OH-. If you add HCl to decrease pH you are still adding Cl- which will raise EC. If you were to add NaOH to lower pH you are still adding Na+ which will increase EC.