Odd pH fluctuation

I am doing a bubble bucket grow and I keep a bucket with tap water and one with extra nute solution ready to add to the buckets as the liquid levels drop.
I keep an aquarium heater and an airstone running in the extra water so that when I add it to the buckets, it will be at a similar temp and have some oxygen in it already.
My local tap water has a pH of 7.4 and a ppm of 295, but after it sits in the water bucket for a couple of days with just the airstone and the heater, the pH goes up to about 8.4.

Why is the pH of the plain water going up?

I remember from science that sometimes running air through a liquid can cause some chemicals to precipitate (like calcium carbonate). Could this be causing the pH to go up?
Should I just let the water sit without air or heat before I use it?
I thought that letting the water sit for a few days would help reduce any chlorine, etc. rather than using it fresh from the tap and just adding pH down before using it.
How do you guys manage the adding of extra water or nutes to your bubble buckets between rez changes?

I hope that this grow works because I am amazed at the difference in size between the soil grown plant and the hydro grown plant. The hydro plant is about 3 times larger and it's only on its 4th week since sprouting.
Thanks for any advice.

The tap water is probably gassing off chlorine and stuff which could make the ph change. That is some pretty hard water if you are talking on a .5 ec scale.

Never worry about or adjust pH in plain water, it will not stabilize. It seems to me you are wasting tons of energy when its not needed. I start with 5gal in my buckets and when it gets down to 1-1\2gal I change it, done. No need to top off water or nutes or keep extras of everything sitting there. K.I.S.S.

295 PPM is WAYYYYYYYYY to high to be starting off with in the first place. If you wanna grow high quality buds that smoke and taste good then you need to stay away from tap water. Some people also like using run off water as well but the best water you can use is reverse osmosis water. Any water dispenser at places like walmart or cvs will give you 5 gal of reverse osmosis water with a ppm of about 5-8 for $1.50, and the 5 gal jugs you buy to fill it are only 6 bucks and they can be reused for years and years. This is somthing you have to do if you want to stabilize your ph. Also like superstoner said, dont adjust ph until about 20 mins after you have put the nutrients in the water and mixed. If you test it right away you will not get an accurate reading because the ph changes haven't completely occurred yet.

There is no reason you should have to use a heater in the water unless its going below 65 degress, and it can be even cooler then that during flower. With a grow light setup you shouldnt have to worry about heating the water unless its 50 deg inside your house which im guessing its not. Lose the heater and start with filtered water. If you use tap water with a ppm of almost 300 then you reading will always be off. Think of it like this-When an artist paints a picture, he doesnt start with a dirty canvas. In hydroponics the water you use is the base for which you grow your plant, water isnt expensive so dont skimp on it. I know its a hell of alot easier walking to the sink but even if it you have to fill three jugs once a week its only about 5 bucks. I can promise if you use reverse osmosis water your ph problem will stop.

I dont know what kind of nutrients you are using but organic nutrients such as Fox Farm do not go well with dwc and that will always give you ph problems. What line of nutrients are you running?

This is what you can do. I don't know your water, but for 5 gallons of plain water in a reservoir, just put in 2 capfuls of PH down and it should be about right in the plain water.

Like superstoner said: Keep it simple.

I've never bothered keeping my gassed off water bucket hot or bubbled. It might sit for 24-48 hours, but honestly I don't think thats long enough for any pathogens to take hold. I mix my dry nutes in a gallon jug and pour it in my bucket with measurements on it. The nutes in the water stabilize the pH at a nice 5.7 and I never have to worry about it later on.

Oddly enough I had the same kinda issue you have but that was before I gassed off my water. I would mix my nutes up straight out of the tap, but 24 hours later it would go from 5.7 to 6.9. As soon as I started gassing my water for 24 hours the pH issue went away.

Thanks for the advice.
I am using canna aqua nutes for the grow, along with some of the canna additives like ryzotonic and cannazyme. I have been feeding about half strength of the recommended amounts.
So far, the roots seem ok. One plant has a bit of discoloration in the roots and the other one seems fine. Both hydro plants are 3 to 4 times bushier than the soil grown plants I have done.
I have some h202 to use as a root bath and will probably do a dip on the dark roots and then add a little to the bucket if it doesn't improve.

I decided to forget the water bucket setup and just use the tap water after it has been sitting for a day and at the ambient temp since it is too much trouble to keep a water bucket stable.
I have no options with the tap water, and I have used it before, so it will have to do.
I did buy a britta water filter that reduces the tap from 7.4/0.3/290 to about 6.3/0.2/190 in pH/EC/ppm.
I use that to prepare the nutes but I just use the pH adjusted tap water for topping up.
I have some cal/mag and also some epsom salts to use, but since the tap water is already so hard, I probably don't need any cal so I may add a little epsom salt for just the mag.
I am trying to find a clear chart of leaf deficiencies with some explanations so that I can monitor the health of the plant as it responds to nutes and conditions.

My grow room is an unheated garage, so the bucket heaters are needed and keep the temps between 17-21C. I even have a small oil radiator in the room to keep the temps ok while the lights are out.