humidity at 65%-82%...... details of the setup below

I have 3 rooms sealed airtight. The temperature is not a problem because the AC works pretty well. I can get it down to 72 degrees or lower. But the humidity is pretty bad.

Why is the humidity so high? The AC drains plenty of water out. Should I have the carbon filter inside blowing air out? Will that solve anything?

How much solution do you feed the plants each time? All that water goes somewhere (into the air)... by either transpiration or evaporation. And the a/c isn't removing all that water in the air... for the 3 rooms... and you have high rh... assuming no other air exchange. You'll either need to do the negative air pressure & exchange as you mentioned... or get a larger dehumidifier, say one of the thermastor models.

Consider enclosing ALL water sources, so they have less of a chance of evaporating. (also helps prevent algae growth in your tanks if they were receiving any light.) Also keep in mind the ideal res temperature is 58 degrees. Anything over 72 will increase evaporation and promote bacteria growth.

you WILL need to lower that before flower otherwise you stand a fairly good chance of getting budrot.

I dont know if this helps humidity alot but consider getting a nutrient that has Silica in it is supposed to help reduce transpiration (protekt is the best)(most companys have there own silica product)